An 18-wheeler is a very large truck for carrying goods a long way, named because it runs on eighteen wheels. In American English this is the picture people have: a high cab in front pulling a long box trailer on the highway. You also hear eighteen-wheeler, big rig, and semi. The front part (the tractor) has two steering wheels, then dual wheels on two axles at the back of the tractor; the trailer has dual wheels on two more axles — that is how you get to eighteen. Though actually, not every huge truck on the road has exactly eighteen wheels; people still call the classic long-distance rig an 18-wheeler even when a count would argue. British English more often says articulated lorry or artic; 18-wheeler sounds American. It is informal, spoken, and a bit colourful — a news reader might say lorry or HGV instead.