I have a bike with 650 wheels, and after going through four (!!) tubes this week, I'm buying some new ones. (Changed my first tube... quite a, um, learning curve.)

I don't understand what all the numbers mean when I go to buy new tubes and tires. The box my tubes came in says...

650 x 18 <--> 23, B1
I understand the 650, and know I need to match that. But what about the 18 and 23?

My tires say
26 x 7/8
I assume I need to match those dimensions exactly? I really don't understand either number, though I suspect 26 refers to inches? (And why are we mixing imperial and metric measuring systems? Agh!)

Hrmm... can anyone help me figure this out?