A peak flow meter measures your  peak expiratory flow - how fast you can force air out of your lungs. It's small, handheld, and very easy to use. They come both in electric and power-free varieties. They're very inexpensive: Mine cost $35. The downside is that they're entirely effort-based, so you have to make sure that you try your hardest to get a high reading every time. Also, peak flow is not the be-all and end-all for telling how you're doing with your asthma. I've had serious attacks that didn't affect my peak flow reading very much.

A spirometer measures the total volume of air you can breathe in or out. It's typically bulkier and requires quite a bit of training to use and interpret correctly. They can be quite expensive: The one at the clinic I go to cost $3000.

Simply due to cost and ease of use, most people prefer a peak flow meter for use with their asthma action plan.