Sea kayaks, sometimes also referred to as touring (*) or expedition (*) kayaks, are the longer, slimmer, often lighter and much more expensive cousins of a typical kayak, often referred to as “recreational”, that one might find in a general sporting goods store like Dick’s.
While some of the sea kayaks are still made of plastic, higher-end models will use fiberglass, Kevlar, carbon fiber or some combination of those. They are often longer and slimmer than a recreational boat giving them less primary stability (i.e. how stable the boat feels when you first get into it) But the same attributes allow these boats to be faster and glide through the water easier to cover longer distances and handle rougher coastal conditions (tidal currents, waves, wind, surf, etc…).
See my Kayaker Journey Progression Model for more differentiation between recreational and other types of kayaking
(*) - to be fair, and depending on who you ask, these terms aren’t interchangeable. Not all “touring” kayaks are considered sea kayaks if they are intended for more sheltered, flat water environments and not all sea kayaks are “expedition” kayaks, which are intended to be long enough to cover distances and to hold enough volume for multiple weeks of food.