A land crab is, as the name suggests, a crab that lives on the beaches above the shoreline, as opposed to one that remains mostly in the water.
JFTR, there are two animals called “king crab” and neither are true crabs. One is the creature more often called the horseshoe crab, the sole living member of Class Xiphosura, and indeed more closely related to spiders and other arachnids than to crustaceans.
The other is, like crabs, lobsters, and true shrimp, a decapod, but is a large spidery creature of the North Pacific. In point of fact, as detailed by Stephen Jay Gould in one of his classic science essays, it is actually a close relative of the hermit crabs. (There’s also a “false king crab” with approximately the same range and ancestry as the [type II] king crab.)