Take a look at the Boston Gear site
www.bostongear.com
You can download their catalog, or look at it online.
They have a good section on gear terminology and design, near the end of the catalog.
The fundamental question about gears is the "pitch angle". Common values are 14 1/2° and 20°. Gears of different pitch angles will not work together, so you need to know what variety you're replacing. This parameter is not easily measured.
Next most important is probably "diametral pitch" (abbreviated D.P.), which is the number of teeth in one inch at the pitch circle diameter. For a 12 D.P. gear standard, a 12-tooth gear would be 1" in diameter, and a 36-tooth gear would be 3" in diameter. As with pitch angle, mating gears must have the same diametral pitch since it determines the width of the tooth and the gap between teeth.
You'll encounter the term "pinion". There's nothing special about a pinion. It's just the smaller gear of any pair.
You'll find "pinion rod" in tool catalogs. These are rods with teeth cut axially. You cut off whatever length you need, then machine off unwanted teeth to form shafts, bushings, or other features. It's easier and faster than buying a small gear and fitting it to the other parts.
- Leigh