This and MASM (apparently) are just different versions. I usually use the official (MS) link. Now MASM is also included in the Windows SDK - if you install it, it contains ML.exe in the bin .
If you just use it to learn assembler, the version of MASM used is not a big deal. Newer versions of MASM add support for 64 bits ( ML64.exe ) and newer instructions as they are added to the x86 instruction set, but more on that. The main differences between different collectors are different dialects. There are 3 main dialects of x86 assembly language: MASM syntax, NASM syntax and Unix style as syntax (also used Borland TASM, but today it is almost dead). MASM, NASM and YASM use the same command and register names, but have slightly different conventions ( dword ptr [blah] vs. dword [blah] , etc.) and completely different macro languages. MASM also has some higher-level constructs, such as .if / .endif , invoke , etc., which are not found in other assemblers. Regardless of whether this is an advantage or not a matter of taste, I personally prefer the NASM style syntax because it is more regular, and I think the macro preprocessor is more convenient to use, but it is a matter of taste.
as is another matter. It uses completely different syntaxes and command names, which differ from the data given in Intel manuals. This is the default value for most Unix variants (as well as for compilers that come from this environment, such as GCC), but is mostly not used outside of this environment. Current versions of GNU as also support Intel syntax, which makes most of the syntactic differences, but as a whole is primarily intended as an auxiliary assembler for compilers rather than a full-fledged macro assembler, so it still has a very limited set of functions compared to MASM or NASM / YASM.