Types of Aerial Lift Platforms
Although there are others (vertical mast/personnel lifts, and push-arounds, to name but two) there are three principal types of aerial lifts:

1. Telescopic boom aerial lift--sits on a swivel base, can be raised to any angle, and then extended straight out like a telescope. The clue is in the name.

2. Articulated boom aerial lift--like its telescopic brother, this also sits on a swivel base, and can be raised to different angles. However, it is articulated, which means that it has a joint that performs the same function as an elbow in a human arm.

3. Scissor aerial lift--its base does not swivel, and is generally the same size as the aerial lift platform that sits upon it. Again, the name describes the mechanism. As the platform rises, and falls, the supporting structure resembles multiple pairs of scissors opening, and closing.

Buying Criteria for Aerial Lifts
When buying (as opposed to renting) an aerial lift, it is important to think creatively about the situations in which it may be used. Perhaps it is going to be mainly deployed for cable work, or painting and decorating, or steel frame erection. But might it have other applications? And how embarrassing and costly will it be if it becomes necessary to rent another aerial lift because the one purchased cannot fulfill an unanticipated role? There is, as always, a trade-off between cost, and flexibility.

Having identified the tasks that a new aerial lift platform might have to undertake, there are five key questions--besides the obvious issues of price, and delivery--that must be asked when making a purchasing decision:

How high must the aerial lift reach?
There arent any hard, and fast rules about how high different sorts of platforms will reach, and aerial lift manufacturers are constantly innovating, and breaking previous limits. However, telescopic boom lifts tend to have by far the longest reach, and the tallest rise to 120 feet. Articulated boom lifts tend to operate in the 30 to 60 feet range--sometimes a little more, while scissor lifts are usually used at considerably lower heights: around 20-40 feet.

Note that it is normal for all measurements to run from the ground to the aerial platforms base. Add six feet for the actual working height.

How accessible is the location that the aerial lift must reach?
There are two parts to this question.

First, what obstacles might prevent the whole lift base, and aerial platform from maneuvering into position on the ground? Where a lift has a boom, that boom will be much longer than the base, and the unit will be unable to turn tight corners. A scissor lift is usually the same size from the ground up. Measure aisles, doorways, and tight spaces where maneuvering might be tricky.

Second, how accessible is the at-height location where workers will actually be operating? An aerial platform on the end of an articulated boom is most flexible because it can be lifted above ridges, and stacks, and into tight corners.

How heavy a load must the aerial lift be able to carry?
A lifts operating capacity is critically important. A 300 pound lift will carry only one person and some light equipment, which may well be insufficient for many types of jobs. It is also possible to find models with lifting capacities of 2,000 pounds or more.

How rugged is the terrain on which the aerial lift must work?
Some aerial lifts are designed to operate safely only on slabs, such as might be found in a warehouse. These cannot be used in exterior or rugged terrain situations, where it is vital to use equipment that is intended for such environments.

How plug-inable is the aerial lifts working environment?
Most aerial lifts require a power source. Those used exclusively in interior locations usually run on electricity. Those used exclusively outside normally have diesel or gas engines. Aerial lifts that have to operate in both environments often have two engines: a gas or diesel one for exterior use, and a propane one for inside.

Sources
Upright.com
Articles.directorym.co.uk
Buyerzone
Goldcoasthilift.com