Docks and piers are very similar but docks are more likely to be floating, while piers have pilings holding them up. Docks can be found in both fresh water and salt water but are more common in fresh water. Some are made just for fishing, and some allow fishing although that's not their primary purpose. Docks can give you access to deeper water than you can reach from the bank, just like piers.
Docks in Fresh Water
If you are lucky there will be a marina or other private business operating a fishing dock near you. These docks are usually fairly large and consist of a covered area with a walkway around the outside and inside. The center of the dock floor is open and you can sit inside and fish through the hole in the floor while protected from the weather. The walkway around the outside gives you a place to fish when it is nice out there.
It will usually cost you a small fee to fish from one of these fish houses but you can be comfortable in all kinds of weather. Protected from the rain, wind, and snow, you can catch fish in comfort — some even have heaters to keep them warm in the colder weather. Most have brush around them to attract fish, and there's usually bait and tackle available, too.
Many fish docks are lighted at night so you can fish all night long. The lights help you see what you're doing and they also attract baitfish, which will bring in the crappie, bass, hybrids, walleye, and other fish that feed on them.
Many parks have a dock near the campground for campers to tie their boats, and you can usually fish from these. Fishing is sometimes allowed at gas docks at marinas as long as you don't interfere with business. And docks at waterside restaurants and other businesses are sometimes available for fishing. Docks in marinas where boats are moored are usually off limits to fishermen unless you own a boat tied up there. Sometimes you can fish from the walkway going out to the docks. Be sure to check with the marina operator before fishing from any docks.
Marinas in salt water also have docks where boats are stored, and the same rules apply as for freshwater fishing. Fishing from walkways is sometimes permitted and some marinas may even let you fish off the ends of the docks. If you do go out on a dock with boats present be sure to respect the property of others so you'll be allowed to come back.
Restaurant and other waterside business boat docks like tackle stores, bars, and warehouses in salt water are more common than in fresh water, and fishing is often allowed from them. The docks where fishing boats tie up can be good for many species, especially if they clean their catch and dump waste fish from their nets around them. And transient docks where boats are tied for short periods of time for loading or unloading, or for campers at public parks, can be another place to get away from the bank to fish.
Due to the constant current and water level fluctuation in many areas as the tide goes in and out, floating docks do not hold up as well. Sometimes in estuaries you will find floating docks with walkways to them and you can often fish from them if they're in public areas.