All Mountain Bike Parts Descriptions

#21) Seat Post Bolt or Quick Release - Basically, it's the bolt that pinches the frame tubing to hold your seat post.
  Most are steel M6 standard allen key bolts, usually going with steel bicycle frames. Aluminum frames have a seat collar to avoid stripping its softer material, and they use a flip lever instead of a regular bolt to tighten the seat post. (called a quick release)  It may be easier for thieves to take your post and seat using the quick release type, but for easy transport they are great. Just don't leave your seat attached to your bike unattended. If you pop the quick releases on your post and wheels, your bike can store in a car trunk quite easily.

Bike Parts Diagram