Sometimes you end up visiting strange sites. This is a question & answer site. The question is:
Obviously, this is not a good question. It is something like "what is the difference between a ham sandwich and butter?".
The answers are not so good either.
"Simple X"? Spelled like this, one would suspect that any method is more advanced. However, big-M is not a replacement for the Simplex method. It is just a simple modeling trick to handle models with an infeasible starting basis. Big-M methods are not used in practice. (Note that there is also a big-M concept in integer programming. There it is used to model implications with binary variables.)
Here is the "expert-verified answer." Basically, the same answer. "Big m" is usually spelled big-M (the M is big).