Road bikes need to remain on blacktop because of geometry and gearing. But mountain bikes are at home on city streets and rocky trails alike. The best way to adapt a mountain bike to any terrain is by changing tires. Traditional knobby tires are used on trails, and street tires are better for roads. Even though the two are interchangeable on mountain bikes, and either one can be used on both surfaces, sticking to each tire's specified discipline yields better performance and comfort.
Knobby tires look exactly as their name implies, with large protrusions around the perimeter of the tire. The knobs can be round, square, triangular or any geometric shape you can dream up, and they come in patterns or simple rows with sharp or blunt tips. They can have any configuration and are sometimes colored. City tires for mountain bikes are smooth, black rubber. They typically have a tread pattern similar to a car tire with a slightly inset, weaving tread line that adds little or no effect to the performance. City tires often have white lettering or white sidewalls that are strictly for aesthetics.
Knobby tires vibrate. They buzz or hum on concrete, blacktop or hard-packed dirt. The noise begins at about 5 mph and gets louder the faster you go. The noise is not noticeable when you're riding on trails or dirt, but on pavement it's substantial and the vibration of the knobs can be bothersome riding on blacktop. Smooth street tires glide easily on cement or blacktop. They are almost completely silent and provide a nicer ride, even on hard-packed dirt. There is no vibration from a city tire, no matter how fast you go.
Performance in Dirt
Knobby tires can outperform any street tire on dirt. They have far better traction and provide float on sand or loose material, keeping the bike moving as the tire stays on top. Street tires do not have float; their smooth surface cuts down into soft material, causing the bike to bog down and making it sluggish or completely immobile. The obvious distinction between the two tires is traction. Knobby tires provide traction on dirt, while street tires do not. One other factor when comparing these two tires on dirt is that knobby tires can run at much lower pressure, allowing them even better traction, while street tires do not perform well at low pressure, and it's not recommended by manufacturers.
Performance on Pavement
Street tires are faster and make the bike handle better on pavement, cement or hard-packed dirt. They run at higher pressure, giving the tire a smaller footprint than knobby tires, which means there is less tire actually touching the road. This gives the bike a lighter, more nimble feel on hard surfaces. Knobby tires run at lower pressure, giving them a squishy feel on pavement; when cornering at higher speeds, the bike can ride up on the sides of the knobs. This makes the bike feel unstable as the knobs bend and twist under the weight.
If your intentions are to use your mountain bike on both dirt and pavement, you can use semi-slick tires -- it's a combination of both tires and the best of both worlds. It has smaller knobs around the outside perimeter of the tire for traction and a smoother tread pattern down the center. Semi-slicks have a moderate to high pressure rating, so you can run them on dirt or pavement. Some mountain bike racers actually prefer this type of tire because it is fast and has a moderate amount of traction for climbing and riding in loose material.