You go to the gym. You see one chick grab the squat rack, load on some 45 lbs plates, and go to work. Nearby, some dude picks up some 60 lbs dumbbells and cranks out on bench. Why didn’t the chick use the dumbbells? What’s that dude got against the barbell? Well, they’re achieving different things. If you are not aware, dumbbells are those weights made to be held in one hand each, and are usually set at a certain resistance, although some are adjustable. A barbell, on the other hand, is much longer, usually gripped with two hands. It’s also generally made for weight plates to be added to each end for more resistance. There are pros and cons to each.
With dumbbells, you are forced to use more of your stabilizer muscles, since each side is not connected, you have to work a little more to keep them even. Because each side is being moved independently, this also emphasizes any imbalances you may have. If you can curl 30 lbs 10 times with your right side, but only nine times with your left, you know what you need to work on. Plus, if you don’t have a spotter or you’re just a beginner that wants to take it to the limit, dumbbells are safer, as you can just drop them to the side (as long as no one is around that might get hit by them).
When you use a barbell, though, you can go much heavier. Try pressing 200 lbs with a barbell and then dumbbells. The latter will be much harder, and once you ARE past the 200 lbs point, good luck finding dumbbells to keep up with you. Plus, barbells are better for training legs, since it’s going to be a bit unwieldy to use separate weights in the squat rack. Besides, try keeping those weights stable when you deadlift. You don’t want those things floppin’ around like a fire hose. Someone could get hurt.
Now you know: dumbbells when you need to fix imbalances or are starting out, barbell when you need to move some heavy-ass shit.