I agree, Jim Bardo's site is an excellent resource. From there, you can see the differences with great descriptions and pictures.
6107 Sheet stamp (perforated on all 4 sides). USPS microprint underneath flag at left.
6108 Coil (perforated on left and right sides). USPS microprint underneath flag at right.
6109 Coil. USPS microprint underneath flag at left.
6110 Coil rolls of 3000 or 10000. See below. USPS microprint underneath flag at left.
6111 Booklet (perforated on 2 or 3 adjacent sides). USPS microprint underneath flag at right.
6112 Booklet. USPS microprint underneath flag at left.
There are perforation shape and measurement differences too but these are the basic ways to identify them.
6108 and 6109 are issued in coil rolls of 100 where the stamps are adjacent, touching each other.

6110 is issued in coil rolls of 3000 or 10000 where the stamps are spaced farther apart on the backing paper, presumably to make it easier for automated machines to separate and apply the stamps to envelopes.
