Nut is a steel or any other material with a tube shape object with a thread inside it. there is a very large verity of nuts used in different industries depending on the technical parameters its needed for
Hex - a six sided nut, also called as a finished hex nut
Heavy Hex - a heavy duty version of standard nut.
Nylon insert lock - a nut with a nylon insert to prevent backing off, also referred to as a nylock
Jam - height of this nut is reduced than a Hex
Nylon insert jam lock - its a nylock nut with a reduced height
Wing - this nuts can be tighten by hand
Cap - a nut with a domed top cover the end if the fastener
Acorn - its a higher type cap nut just for the purpose of appearance
Flange - its a nut with a build in washer like flange
Tee - commonly used for wood to be driven in to as a thread
Square - a four sided nut
Prevailing torque lock - a non reversible lock nut used for high temperature application
K-Lock or kep - a nut with a n attached free spinning external tooth lock washer.
Coupling - this is used to connect threaded rods into each other
Slotted - slotted nuts are used in conjunction with cotter pin on drilled shank fasteners to prevent loosening commonly used in hydraulic jacks.