Dirt bike & street — 120/80-19120 mm wide, a sidewall 80% as tall as it is wide, for a 19-inch rim. An R (180/55R17) means radial.
ATV & UTV — 25x8-1225 inches tall, 8 inches wide, for a 12-inch wheel. 6PR is the ply rating — higher is tougher.
TT or TLTube type or tubeless. Match what your wheel runs.
Front and rear differMost machines run a different size front and rear — read the sidewall of the tire you're replacing.