If you already have a shower or bathtub door, some of these will work with your current frame, or lack thereof. If you wish to get a shower door, choosing how you want to enter the shower space is a big part of the selection criteria.
Hinged: Set on a large hinge and opens by swinging in one direction. Sometimes reversible.
Pivot: Swings in and out of the shower. Offers greater flexibility.
Sliding: Set on tracks, it slides along a frame guide. Easy to remove.
Double Sliding: Also on track guides, this is designed for large showers. Both sides slide.
Fixed: Only one half of the shower has a glass partition that doesn't move. The other half of the shower is open.
Folding: A multi-sectional door, with hinge points set in frame tracks, that folds when opened. Similar to an ornamental japanese screen.
Once you have picked your hardware, how you want the shower or bathtub door to open and what, if any, frame you want, now it's time to pick your glass.