Whistles are long cylindrical instruments, with six holes (none on the bottom), just over a two octave range, meant to be played by a single player, unless of course it is around the low Ab range, or lower (in which case, the holes are too large and far apart for a single player to cover them). Whistles generally come in two varieties: pennywhistles (aka tin whistles), and low whistles (these are exactly like pennywhistles, only larger and lower in pitch). Recorders are not whistles, although recorders and whistles share many things in common: fipples, open holes. . . . Whistles can be made out of almost any hard material (plastic, metal, cane, etc.)
Typically, whistles are used in Celtic music - however, they tend to make appearances elsewhere (soundtrack music, music of almost any genre except for classical, etc.) and are often mistaken for flutes, despite the characteristic differences, though all whistles do not sound precisely the same.