Lifts client up stairs with seat mounted to track attached to stairs.
Sometimes referred to as chair lifts, stairway lifts, stair glides or stair chairs.

Questions and considerations associated with the client
Weight and height of user of stair lift or chair lift?
Ability to transfer to seat of stair lift or chair lift?
Seat height at bottom of stairs is an important point if client is short.
If you have lower back pain and a soft start and stop are important then a DC powered stair lift unit maybe more appropriate.
If user is tall then seat depth of the stairlift is important.
Is the client able to bend his or her legs?
Paralysis or difficulty of use of one or more hands while operating the lift may be a consideration.
Will someone be there to assist with chair lift use?
Evidence of cognative difficulties?

Questions and considerations associated with the site or place of installation
Stairs straight or curved?
If curved, shape of curve?
Stair lift on left or right of stairs looking up from bottom?
Width of stairs in inches?
Width of stairs constant?
Is there an existing hand rail?
Rail mounts to stair treads, not to wall. Stairs must be in sound condition.
Stairway must have adequate lighting at top and bottom of stairs.
Ceramic or hardwood floors at the bottom of the stairs.

Other pertinent questions and considerations
If your stairs are narrow (less than 37") than the track distance from the wall will be a key issue. This factor is also dependent on the height of the user and their leg length.
If you have difficulty transferring to the seat of the stair lift then seat height will be a key issue.
Stair lift track must be allowed to come all the way to the top landing and bottom landing for safe transfer to the seat of the stair lift.