Sea ice and ice sheets differ fundamentally in how they form, where they're located, and their composition.
Sea ice is frozen ocean water that forms, grows, and melts within the ocean itself. Because it originates from saltwater, sea ice contains trapped pockets of brine (salt), though older, multi-year sea ice becomes progressively fresher as brine drains out over time. Sea ice is generally thin, typically ranging from a few centimeters to a few meters thick. It's highly dynamic, expanding dramatically in winter and shrinking in summer — this seasonal cycle is especially pronounced in the Arctic and Antarctic. Sea ice floats on the ocean surface, and because it's already displacing water, its melting does not directly raise sea levels (similar to how ice cubes melting in a glass don't overflow it). Sea ice is found in polar and sub-polar seas, such as the Arctic Ocean and the waters surrounding Antarctica.
Ice sheets, by contrast, are massive, continuous masses of glacial ice formed from compacted snow accumulating on land over thousands of years. They are composed of fresh water, not salt water. Only two ice sheets exist today: the Antarctic Ice Sheet and the Greenland Ice Sheet, and together they hold the vast majority of the world's fresh water. Ice sheets can be kilometers thick and cover enormous land areas. Unlike sea ice, ice sheets flow slowly under their own weight, often draining into the ocean through outlet glaciers and ice shelves. Critically, because ice sheets rest on land rather than already floating in the ocean, when they melt or when ice breaks off (calving) and enters the sea, they add new water to the ocean and directly contribute to global sea-level rise — making them a major focus of climate change research.
In short: sea ice is thin, salty, ocean-based, and seasonal with limited sea-level impact, while ice sheets are thick, freshwater, land-based, and their loss directly raises sea levels.