Sure Erin!

650b and 700c are two different wheel sizes for road bikes.

700c is the standard size.
650b is sometimes used on smaller bikes to make them more proportional (solves some of the issues with toe overlap and frame geometry). They are smaller in radius (hub to rim). Some smaller folks love them, some prefer the standard size (numerous reasons on both sides).

Try this thread for an idea of the benefits of each type: http://forums.teamestrogen.com/showt...ght=wheel+size

Toe overlap refers to your toes overlapping the front tire when your foot is in the front of the pedal stroke (typically 3 o'clock) such that if you turn your tire you are in danger of rubbing your foot with it.

If you are aware of the possibility then generally you can avoid it (you don't turn road bike wheels that much very often anyway). But it can cause low speed turning issues if you aren't careful, or a nasty crash if you happen to turn your wheel into your foot at a decent speed.

Some people learn to live with varying degrees of this and never have a problem (me), others choose to pick bikes very carefully to avoid it entirely.