Although not overly complicated, a shower drain is more than just a hole in the bottom of your shower. Shower water can't simply be released back into the water table, so the drain has to connect to a sewer while preventing sewer gases from escaping. Moreover, the drain opening must be watertight so that water goes down the drain without seeping under the base of the shower where it can rot the floor.
The Shower Drain Trap
Piping for a shower drain includes a trap like the ones you see under your sinks. While sink traps are usually P-shaped, some shower traps are S-shaped so that water drops vertically away from the trap when it clears the S-curve instead of being directed through a horizontal length of pipe. The bottom of the trap holds a pool of water that provides a seal against the incursion of sewer gases and vermin through the shower drain. The bottom of a shower S- or P-trap is a natural repository for hair, soap and debris that can form clogs.
When you take a shower and water begins flowing down the drain, it forms a vacuum in the drain line that can slow the water down. To keep the water moving, the drain line is vented with a pipe whose sole function is to allow air into the drain line. It connects to the shower drain at the point where it turns vertically downward and rises to connect with a main vent stack that extends through the roof. Without venting, the vacuum can become strong enough to siphon water out of the drain trap, rendering it useless.
The Shower Base and the Drain Seal
If water remains standing on a shower base, a soapy film can form that creates a slipping hazard, so the base must be designed with a downward slope toward the drain at all points. This slope is built into prefabricated shower bases, but if you build your own shower stall, you must make it yourself. You must also carefully seal the strainer to the shower base to prevent water from seeping under the base where it can do serious damage. When you make a base for a tile stall, you usually make this seal with vinyl underlayment.
Keeping the Shower Drain Clear
Shower drains are susceptible to clogging from hair, which collects around the strainer and in the curve of the trap and in turn collects other debris. You can't always clear a hair clog with a plunger because hair tends to cling, and if you can't remove the strainer, an auger isn't an option. Chemical drain clearing compounds can be effective, but they aren't very good for your pipes. A good strategy to keep your drain clear is to periodically pour an enzyme-based drain cleaner down the opening. It eats through clogs slowly but is safe for the pipes and the water table.