Socionics superficially looks exactly as MBTI. 8 functions, 16 personality types. There's even a table, perfectly corresponding socionics types with MBTI types. Then what makes socionics and MBTI importantly differ?
Both socionics and MBTI are a try to dig deeper on the path of Carl Gustav Jung, who observed empirically (both from his work as a psychiatrist and from his travels across the globe) that all human brains displayed common cognitive descriptions, which Jung described by two energy-attitudes, two perception functions and two judgment functions.
About Jung (about 100 years ago - his milestone book about cognitive types
was published in 1921)
The energy attitudes are introverted - the energy flows outward from the
inside stimulation; and extraverted - the energy flows inward from the
The perception functions are intuition - a misnomer, with intuition you
take information from a 3,000 ft view and make connections from this - and
sensation - you take information from your senses and like to get into
details, like with a magnifier glass.
The judging functions are thinking - you make your decisions based on
logic - and feeling - a slight misnomer, you make your decisions based on
your subjective set of values (N.B. objective and subjective are commonly-used
words in C G Jung's books).
Following Jung, we all have a piece of everything in us, but we also have a
preference for each dimension - energy management, taking information,
making a decision.
Jung described a set of 2x2+2x2=8 cognitive functions: introverted thinking,
introverted feeling, extraverted thinking, extraverted feeling, introverted
intuition, introverted sensation, extraverted intuition and extraverted
sensation - also known as Ti, Fi, Te, Fe, Ni, Si, Ne and Se. Yes, N stands
for iNtuition, as i stands for Introverted.
As mentioned in the comments, the MBTI was developed in the US - Katharine Briggs met Jung in Switzerland and her daughter Isabel Myers-Briggs was raised in a family/friends/colleagues environment that talked about cognitive types and she followed her own path, about 35-to-50 years after Jung.
About Briggs and Myers (about 50 years ago)
They described the structure of the preferences of the cognitive functions:
sixteen combinations rephrase Jung's preferences, with the three axis from
Jung (energy orientation of the dominant function, perception function,
judgment function) and with a fourth axis, describing whether the
extraverted function among the main two functions used by a profile
is either a perception function or a judgment function.
This descriptions fits into four letters (E or I, S or N, T or F, J or P)
that describes the preference of a cognitive profile for two Jungian functions.
Every profile has an inward function and an outward function, every profile
has a perception function and a judgment function. With a main (dominant) and
an auxiliary (auxiliary) function, this makes 16 combinations of letters,
and 16 archetypes to describe common traits of a profile.
The Myers-Briggs Type Indicator (or MBTI) is a questionnaire that tries to
figure out whether your preference is E or I, S or N, T or F, J or P. Its
main weakness is that this way of typing is not accurate at all - though it
had a huge success in business. This was a weakness from the science viewpoint,
as the MBTI never improved upon itself, by trying to describe with better
accuracy the nature of our cognitive functions (which was Jung's goal).
Now, we turn to the former USSR, where other people followed the path of Jung,
too. They founded socionics, which is an extrapolation of Jung's work to describe
how the 16 profiles communicate and interact with one another.
About socionics (about 50 years ago - USSR)
Socionics also developed a fourth axis beyond Jung's three axis, but do not
be misunderstood about it, as it is not the same fourth axis as in MBTI.
In socionics, the fourth letter describes whether the dominant function of
a profile is a perception function (p) or a judgment function (j).
However, while MBTI focused on typing with four letters, socionics kept
working both on the four-letter code and on the description of the Jungian
functions. For the four-letter code, socionics described one-letter
archetypes like in MBTI, but also two-letter and three-letter archetypes.
About the Jungian functions, socionics described how a given Jungian function
(e.g. Introverted Thinking) could be described by a slightly different archetype
depending on its rank within the eight functions - while MBTI described
mostly the first two or the first four functions of a profile, socionics
tried to describe the eight functions of a profile, by decreasing order of preference.
Trying to describe a cognitive profile by an order of functions leads us to
figuring out how stress states may alter our use of our preferred functions.
This topic was mentioned by Jung (through the concept of Shadow), it is hardly
described in MBTI (not much developed either is the topic of mastering our
functions as we grow older - there are a few life stages when we care
differently for our preferred cognitive functions), while socionics described
a stress model called Model-A (this is a first step on our journey).
Wrapping things up - until you ask for more?
When MBTI and Socionics speak (about) the same with a different vocabulary:
MBTI - Socionics - Jungian functions (descending order)
INTP - INTj - Ti Ne Si Fe Te Ni Se Fi
ENTP - ENTp - Ne Ti Fe Si Ni Te Fi Se
INFP - INFj - Fi Ne Si Te Fe Ni Se Ti
ENFP - ENFp - Ne Fi Te Si Ni Fe Ti Se
ISTP - ISTj - Ti Se Ni Fe Te Si Ne Fi
ESTP - ESTp - Se Ti Fe Ni Si Te Fi Ne
ISFP - ISFj - Fi Se Ni Te Fe Si Ne Ti
ESFP - ESFp - Se Fi Te Ni Si Fe Ti Ne
ENTJ - ENTj - Te Ni Se Fi Ti Ne Si Fe
INTJ - INTp - Ni Te Fi Se Ne Ti Fe Si
ENFJ - ENFj - Fe Ni Se Ti Fi Ne Si Te
INFJ - INFp - Ni Fe Ti Se Ne Fi Te Si
ESTJ - ESTj - Te Si Ne Fi Ti Se Ni Fe
ISTJ - ISTp - Si Te Fi Ne Se Ti Fe Ni
ESFJ - ESFj - Fe Si Ne Ti Fi Se Ni Te
ISFJ - ISFp - Si Fe Ti Ne Se Fi Te Ni
I hope this helps!
Socionics was developed independently of MBTI. The creator of socionics (Aushra Augusta) used Jung as a starting point (as did MBTI) but said that in the process his definitions "were changed beyond recognition" (The Socion). She also incorporated the concept of information metabolism from a psychologist named Antoni Kempinsky, so that the functions are not only vague collections of behaviors as in Jung, but objective categories we use to describe reality.
MBTI's functional model is not very well-developed; it was initially developed in terms of the four letters/dichotomies and only recently have people attempted describing how types use all eight functions (Beebe), perhaps influenced by socionics. In socionics the functions were defined very early on, in terms of Model A.
Socionics also has a theory of relationships between types, which is absent from MBTI. Socionics makes a distinction between strengths and values, whereas in MBTI it is not clear that you can value something, yet be bad at doing it for yourself. This is the concept underlying duality, the relationship where you have similar values yet can help each other with your weaknesses in the best way.
To get into the specifics, if you look at the basic definitions of the functions (in socionics called elements of information metabolism), there are many differences. Si in MBTI has to do with tradition and reflecting on the past, whereas this is more of an Ni theme in socionics. Te in socionics is all about pragmatism whereas it's described as "external laws and rules" in MBTI, more of a Ti+Se theme in socionics. Fe in MBTI is described as "seeking harmony" which is really more about Fe+Si (or Fi+Si) and doesn't describe EIEs who have Fe as a leading function. (Notice here how element pairs are also significant in socionics, not so much in MBTI.)
The dichotomies are vaguely similar, but none of them are the same, really. There is a widespread idea that sensors are unintelligent (and unworthy of interacting with the superior intuitives) in MBTI while this is not true in socionics - sensors and intuitives have different areas of intelligence and in fact depend on each other. The J/P dichotomy might be the most different though - Perceiving sounds a lot like Ne/Si valuers in socionics, who prefer to stay open to new information and options rather than committing to a single one and pushing for that (Ni/Se).
All in all, there is no direct translation between types, only correlations. Other types like ENTJ might be something like SLE in socionics.
If you're learning socionics the best practice is to completely keep it separate from MBTI in your mind. Trying to compare them usually ends up confusing beginners more than it helps.
Lytov, D., & Lytov, M. (2005, September 30). Retrieved from Lytov, D. (2005). Introduction to socionics. https://www.socioniko.net/en/articles/lytovs-intro1.html