First let me point out that Mycoplasma is not the
same as Mycobacterium.
In plain language, mycoplasma is a name we call
a group of bacteria that has no cell wall ( that
other normal bacteria have ). These bugs are
born naturally without cell wall.
You can manipulate a normal bacteria, so that
its cell wall is removed. These wall-less
bacteria are called "L-forms".
Mycobacterium is a name of another group of bacteria.
These bugs have cell wall althought, a bit different
than that of normal bacteria. It has a lot of fatty
compounds in it. Some Mycobacterium can cause disease
such as tuberculosis.
I think you should check out some basic book
on Microbiology. I found that Tortora, Funk, and Case
Microbiology is a good start.
Hope my two cents help.