It's impossible to describe because there are thousands of strains of marijuana, and different strains have different smells. For the most part though, marijuana has a unique "thumbprint" to it.
I can't really describe it, but when people say it smells like skunk, that's probably the closest they can get to it iwth actual words, and they're really talking about how it smells BEFORE it's smoked.
It's like trying to tell someone who doesn't know what fresh-baked cookies smell like what fresh-baked cookies smells like. You just know.
Basically, marijuana has no smell that I can relate to something most people would be familiar with. It's distinct. It DOES NOT smell like tobacco though. It smells wonderful :D
If I could put it into words, it would be an oily herb smell. It lingers more than tobacco smoke in the air, and you'll know immediately that you're not smelling tobacco. Of course, we're talking about SMOKED marijuana, and not cured or fresh marijuana, which has hundreds of thousands of unique, unknown words to describe it (think of wine, and how many ways there are to describe the smell).
The word "pungent" comes to mind, and that relates back to what I was saying about smelling like skunk. It's not naseaous like skunk, but it's a word used by smokers to describe both the bud (marijuana flower) and smell, so what they really mean is pungent.
Some smell spicy, some like citrus, some like flowers, some have earth metal overtones, some like dirt, some smell like grass, some like burnt popcorn, some like fresh-mowed grass, etc.
Basically, it's like nothing you have EVER smelled before.