Given how responsive a kayak is to the slap of waves and the pulse of the current, you
can quickly waft away from your desired zone even if you're doing your best to counteract drift with your paddle. Anchors (and anchor trolleys) give you the means to stabilize your position on open water.
There are several basic forms of kayak anchors to choose from. One is the classic fixed or grappling-hook anchor, which sinks to the bottom and latches to the substrate. Often the teeth can fold in when the anchor's not being used, making it all the more compact. Those used for kayaks are commonly 1.5 or 3 pounds, although heavier models are available.
Sometimes at sea or on a lake you may simply want to brake your kayak against the current rather than actually tether it to the bottom. In this case a drogue or sea anchor (also called a drift sock, drift anchor, or drift chute) is a fine, lightweight choice--a parachute-like cloth deployed by a rope from the kayak that inflates to create drag and check your drift. A drift sock's also handy when the water depth is too great for a grapple-style anchor.
Another alternative to a traditional anchor is a stake-out pole, useful in soft substrate: Nothing more than a sturdy rod with one pointed end, it's driven into the bottom and attached to the kayak through the scupper holes, an anchor trolley, or via a line.
An anchor trolley gives you the freedom to shift the position by which your kayak's anchored. Through its pulley system connecting bow to stern you can completely reverse the orientation of the boat or cant it against the current. For an angler in particular this is a fantastic accessory, allowing you to cast broadly across an area and quickly adjust your bearing to better land a catch.
Don't set an anchor in tumultuous waters: You don't want to be so tethered as to risk swamping your kayak. And remember to always have a knife on hand so you can cut an anchor line if need be--buying a new anchor is a much better deal than risking your safety.