Bicycle frames are constructed from materials which place emphasis on being strong, lightweight and stiff to aid power transfer. Aluminium, titanium, steel and carbon are widely used in all levels of frame production.
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 manoeuvrable and have quicker acceleration.
Wheel sizes are defined by the country that first popularised 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.
A frame is effectively the chassis of your bike. Many enthusiasts buy a frame then choose their ideal components to build a bike. Mountain bike frames are usually sold without a fork because most riders want to choose a suspension fork separately. Road bike frames usually come as a frame and fork designed to work together.
Frame design and construction varies enormously. Aluminium dominates the lower price categories with carbon increasingly dominating as prices rise. Top quality steel frames still have their fans because they usually exhibit a more forgiving ride feel than aluminium or carbon. Lightweight titanium frames are popular in higher price categories because of their perceived resilience.
Listed frame sizing varies a lot between manufacturers, with some opting for a simplistic 'S, M, L or XL' description and others quoting the seat tube length. Bear in mind that it's often the top tube reach that's more crucial to achieving a perfect fit. Mountain bike and road frames differ in size because the reach to the handlebar is different and you need more standover room on an MTB.