The seed coat develops from tissues (called integument) originally surrounding the ovule. The seed coat in the mature seed can be a paper thin layer (for example, peanut) or something more substantial.
The seeds of angiosperms are contained in a hard or fleshy (or with layers of both) structure called a fruit. Gymnosperm seeds develop "naked" on the bracts of cones, although those of the yew have a fleshy coat called an aril. An example of a hard fruit layer surrounding the actual seed is that of the so-called stone fruits such as the peach.
Plants have evolved many ways for their seeds to disperse and spread the population. Some seeds are attached to feather-light fibre parachutes that may be blown by the wind. Others have prickly burrs or spikes that will attach themselves to a passing animal's fur so that the animal will carry them away. Seedpods are often designed and shaped so that the seeds are flung away from the parent plant with great force when the seedpod springs open. And lastly, many seeds are contained within a sweet and juicy fruit that invites animals and birds to consume it. These seeds have a tough protective outer-coating so that while the fruit is digested, the seeds will pass through their host's digestive tract intact, and grow wherever they fall.