Each type of integer has a different range of storage capacity

Type     Capacity

Int16    (-32,768 to +32,767) or (−2^15 to +2^15−1)

Int32    (-2,147,483,648 to +2,147,483,647) or (−2^31 to +2^31−1)

Int64    (-9,223,372,036,854,775,808 to +9,223,372,036,854,775,807) or (−2^63 to +2^63−1)

As stated by James Sutherland in his answer:

int and Int32 are indeed synonymous; int will be a little more familiar looking, Int32 makes the 32-bitness more explicit to those reading your code. I would be inclined to use int where I just need 'an integer', Int32 where the size is important (cryptographic code, structures) so future maintainers will know it's safe to enlarge an int if appropriate, but should take care changing Int32 variables in the same way.

The resulting code will be identical: the difference is purely one of readability or code appearance.

The two are indeed synonymous; int will be a little more familiar looking, Int32 makes the 32-bitness more explicit to those reading your code. I would be inclined to use int where I just need 'an integer', Int32 where the size is important (cryptographic code, structures) so future maintainers will know it's safe to enlarge an int if appropriate, but should take care changing Int32s in the same way.

The resulting code will be identical: the difference is purely one of readability or code appearance.

Between int32 and int32_t, (and likewise between int8 and int8_t) the difference is pretty simple: the C standard defines int8_t and int32_t, but does not define anything named int8 or int32 -- the latter (if they exist at all) is probably from some other header or library (most likely predates the addition of int8_t and int32_t in C99).

Plain int is quite a bit different from the others. Where int8_t and int32_t each have a specified size, int can be any size >= 16 bits. At different times, both 16 bits and 32 bits have been reasonably common (and for a 64-bit implementation, it should probably be 64 bits).

On the other hand, int is guaranteed to be present in every implementation of C, where int8_t and int32_t are not. It's probably open to question whether this matters to you though. If you use C on small embedded systems and/or older compilers, it may be a problem. If you use it primarily with a modern compiler on desktop/server machines, it probably won't be.

Oops -- missed the part about char. You'd use int8_t instead of char if (and only if) you want an integer type guaranteed to be exactly 8 bits in size. If you want to store characters, you probably want to use char instead. Its size can vary (in terms of number of bits) but it's guaranteed to be exactly one byte. One slight oddity though: there's no guarantee about whether a plain char is signed or unsigned (and many compilers can make it either one, depending on a compile-time flag). If you need to ensure its being either signed or unsigned, you need to specify that explicitly.

https://golang.org/pkg/builtin/#int

int is a signed integer type that is at least 32 bits in size. It is a distinct type, however, and not an alias for, say, int32.