Bicycle wheels come in a wide range of sizes from 12" wheels found on smaller Kids Bikes to larger 29" Mountain Bike and 700c Road Bike wheels. Larger wheels are normally faster and more stable at speed and their larger contact patch improves grip too. Smaller wheels are normally lighter, more maneuverable and have quicker acceleration.
Wheel sizes are defined by the country that first popularized the style of bike that used them - so you'll see some imperial and some metric measurements listed. Wiggle includes the ISO standard in brackets after each for ease of comparison.