The b-barrel (left) is  a closed, circular b-sheet composed of membrane spanning b-strands of 8-12 residues each. Known b-barrels have between 8 and 22 strands. This example is the 8-stranded OmpA from E.coli. 

The a-helical bundle (right) is exemplified by the protein bacteriorhodopsin. Known a-helical membrane proteins  have from 1 to a few dozen membrane-spanning helices. Individual helices are typically 18-26 residues long