Ethane is a linear organic molecule, C2H6. Because it is a symmetrical molecule, no matter what hydrogen you remove to form an alkyl group, you get the same alkyl group.
Propane is also a linear organic molecule, C3H8. Now you have two choices where you remove a hydrogen atom to form an alkyl group. If you remove any of the three hydrogen atoms on either end carbon, you get exactly the same alkyl group (called a propyl group). However, you could also remove one of the two hydrogen atoms bonded to the central carbon atom. If you do this, you get a different alkyl group (called an isopropyl group).
So, because you have both end and interior hydrogen atoms bonded to carbon atoms you can form more than one alkyl group.
As the number of carbon atoms in an alkane increase you can get more and more different possible alkyl groups forming.