choosing a tread..

SLICK TIRE

Many bike tires made for street use are slick. The tires appear almost smooth with a barely perceptible tread pattern. These are mainly designed for city touring or for commuters. 

SEMI-SLICK TIRE

Somewhere between knobbies and slicks, these tires are designed with a smooth center, for minimal rolling resistance and faster acceleration, and aggressive treads on the side to help with cornering.

INVERTED TREAD 

Tires designed with an inverted tread have less rolling resistance than those with any type of regular tread, but offer more grip—and rolling resistance—than slicks. Use these tires if you suspect you might venture off asphalt or ride on roads with lots of ruts and potholes.

KNOBBY TIRE

Different knobby tread styles are designed for specific trail situations.

• Smaller knobs are faster and suitable for smooth singletrack.
• Taller knobs offer more grip in technical terrain like roots and rocks.
• Wider tires with sturdy paddle-like knobs are best for soft trail conditions.
• Knobs that are wider at the base will corner better on hardpack.
• Tires with tall, widely spaced knobs offer versatility in loose and hardpacked conditions.
• Tires designed for mud have widely-spaced knobbies so that mud sheds from the tire. 

- www.rei.com