why is it some of my roms work and some don't

for example i have some roms that are listed as Final fantasy 5 and it wont load on my supernintedo emulator but other roms that is have like final fantasy 4.smc will work.
What does the smc mean and how did i get that so it worked
thank you

 

i had the same problem so i just deleted my roms and found new ones here http://www.romscentral.com/gbasha006.htm lot of pop ups but really good roms