According to Wildlife Britain, stoats are bigger and longer than other weasels. Stoats have black coloring on the tips of their tails, which are proportionately longer than those of other weasels, and a coat that turns completely white in winter. Stoats are called ermines when in their white winter coloring.
According to Wikipedia, stoats are actually a kind of weasel. Stoat fur, especially in ermine white, is prized by the fur trade. Stoats were brought into New Zealand in the late 19th century to control the rabbit population, but they ended up wreaking havoc on the local bird population instead.