BMX Seatposts - Sattelstange + Co.

Seatpost types:
Rail - compatible with seat that have a rail on the bottom. Can be adjusted in many ways.
Pivotal - compatible only with pivotal seats (there is a bolt in the seat) and a thread in the middle of the seatpost. The seat angle can be adjusted.
Stealth Pivotal - a modern pivotal seat - there is no bolt hole on the top of the seat but it is under the seat and the seatpost has a hole on the side too. The seat angle can be adjusted.
Tripod - Most simple form of seats, has only one bolt and the angle can have only 2 settings.

Length: 
Small - under 100mm
Medium - between 100-250mm
Long - longer than 250mm

Pro Tip: the normal pivotal seats are also compatible with stealth pivotal seatposts but the bolt will be outside the seatpost. But it works.