The size of cassette cogs and chainrings are measured in the number of teeth. So 42x23 means you're chain is in the 42-tooth chainring up front and the 23-tooth cassette in the back.

Easier pedaling = smaller chainring (front) and larger cassette cog (back). Opposite for harder pedaling.

A long time ago, I sat down and counted teeth on chainrings and cogs, and it helped clarify things, since I'm more image-oriented than number-oriented.