choosing a mountain bike handlebar..

Handlebars come in a variety of types designed for particular types of riding.

Drop

Drop handlebars, as used on road or track bicycles, may have a shallow or deep drop. Drop bars may have one or two longitudinal indentations so that the brake and shift cables protrude less when they are wrapped under the bar tape. They may also have a flattened (ovalized) top section to provide more comfortable support for the hands.

Syntace Racelite 2014, Drop Bar, Medium, Black 42cm

Track

Track drop bars are a variation of drop bars designed for the typical riding positions of track bicycle racers. Track drops are characterized by large, sweeping ramps, effectively precluding the top and brake hood hand positions, but promoting the rider's use of the ends, or "hooks". Track bars are designed for use without brake levers, but recently experienced a surge in popularity on use with fixed gear bikes, and as such are have been adapted to fit levers and hand positions.

Nitto Track 40cm 25.4 Alloy Silver bar

Touring or Trekking

Sometimes referred to as "Butterfly" bars, these are commonly encountered in continental Europe. They typically consist of a broken figure-of-eight arrangement mounted horizontally on the stem. This style of bar allows the rider to remain relatively upright while at the same time providing a wide range of hand positions for comfort on long duration rides.

Pyramid Touring North Road Alloy Bicycle Handlebar

Ergo or Anatomic

The shape of the drop may be a simple, traditional curve, or it can have a flat spot (straight section) which some riders find to be more comfortable for their hands. These bars may be described as ergo or anatomic.

Easton EA50 Ergo Road Cycling Handlebar - 31.8mm

Drop-in

At one time, manufactures and racers experimented with drop-in bars that had an additional extension in toward the head tube at the rear end of the drops. This was intended to offer an even more aerodynamic position, due to low and narrow placement of the hands, than just the drops, while still remaining legal for mass-start races. Their popularity has since waned.

Flat or Riser

"Flat" or "riser" bars are the standard handlebars equipped on mountain bikes, and recently on fixed gear bicycles. Flat bars are comprised of a nearly-straight tube, slightly bent toward the rider. Risers are a variation in which the outer sections of the bars rise from the center clamp area. Flat and riser bars may be appended with bar ends, providing more hand positions.

Ritchey Comp Flat MTB Handlebar, 580mm 25.4mm Black

Upright or North Road

North Road bars are swept back toward the rider, with each grip ending nearly parallel to each other and the bike. This type of bar was equipped with three-speed and single speed Raleighs, Schwinns, and other three-speed bikes well into the 1980s, as well as various European utility bikes and roadsters. They have recently enjoyed a resurgence in popularity on some hybrid bicycles, city bikes, and comfort models. They are also known as "Townie", "Tourist", or "Comfort" bars.

Triathlon or Aero

Triathlon bars or aerobars include various styles of aerodynamic handlebars for racing bicycles and particularly time trial bicycles. Included are narrow, bolt-on extensions that draw the body forward into a tucked position, pursuit bars that spread the arms of the rider but drops the torso into a slightly lower position, and integrated units that combine elements of both designs. These are commonly used in triathlons and time trial events on road and track. However, they are illegal in most mass start road races or any other event where drafting is permitted because, while aerodynamically advantageous, they tend to draw the hands away from brakes, make the rider slightly more unstable on the bike, and can be dangerous in the event of an accident.

Profile-Design T2 Clip-on Aerobar

Pursuit

Pursuit bars, or Bullhorn handlebars, curve forward away from the rider. They are often used with dedicated triathlon bars and are also popular, by themselves, on track, single-speed, and fixed-gear bicycles.

- Wikipedia