Back in the olden days, seatposts came in one shape only. If you wanted to alter the fore/aft position of your saddle beyond the rails you needed a custom geometry frame with a different seat-tube angle.
These days, you can get seatposts with a variety of laybacks from inline to way back and even some forward ones. Few people need resort to custom geometry frames to set their saddle with respect to pedal position.

The starting point for pedal->saddle relation is Knee Over Pedal Spindle or KOPS. From this standard relation, you can adjust back and forward to suit your own body shape, power flexibility and position. More here.
I prefer to note this relation by using [x,y] coordinates (cartesian) rather than include seat-tube angles (polar coordinates). I use the BB as [0,0]. By using cartesian coordinates, you can replicate most positions on most production frames of the correct size.

How much layback do you need? Do you know your current riding position? Do you have a bike setup the way you like it at the moment?

Inline posts are generally easier to adjust for level than trad single bolt clamps. Once you yank apart the two ridged surfaces, you loose the original position.
Some posts have single forged construction, others are glued from several bits. Carbon posts can be made in one or glued from components.
Ultralight posts seem to suffer more (or more public) failures than mid-range ones with thicker, but lower grade materials.