Mother 1 + 2 is a GBA Cartridge released so that people could play both of them. It holds all of the content that Mother has, and all of the content that Mother 2 has. The only difference is that a few glitches are fixed in these. This is also the most common way to obtain an English Mother patch. If you played Earthbound, and then imported this, you would notice that there are differences in the graphic design. Not much, but a small amount.
Related ThreadsWe should liven up the MOTHER 1+2 Fourm - last post by Symphonic Abyss @ Mar 20, 2009
Mother 1 & 2 info thread. - last post by @ Mar 25, 2007