Assuming you are referring to cameras, (surveyors use laser & ultrasonic
rangefinders for measuring range), the term mainly relates to a class of cameras
which were mainly popular before and after WWII.  In the simpler models, the
rangefinder had a separate viewing window, and by turning a knob or moving a
lever, two images of your subject could be made to coincide.  The distance from
camera to subject could then be read off a scale and transferred to the lens.
CRF (Coupled rangefinder) cameras automated this process by arranging for a cam
on the lens to move the optics of the rangefinder directly, so when the two
images coincided the lens was automatically in correct focus.  The next step was
to arrange that the mechanism worked with a range of interchangeable lenses as
in the Leica rangefinder cameras.  And the rangefinder window was combined with
the viewfinder window, making for speedier use.  Whilst shutters could be of the
Compur between lens-element design, the term normally refers to the Leica focal
plane shutter with interchangeable lenses.
The benefits are that particularly with wide aperture wide-angle lenses, it is
possible to get accurate focusing, which can be difficult using the SLR method.
The lack of a reflex mirror makes the camera quieter than an SLR, and the Leica,
in particular, was favoured by some of the exponents of candid street
photography (such as Cartier Bresson) for its quietness, and general
un-obtrusiveness.  (Rangefinder cameras were made in a range of film & plate
sizes, but
They are still very popular with their adherents for the advantages I've listed,
but I certainly haven't see many in use recently.
Compared to the average film SLR of a few years ago, rangefinder bodies were
slimmer, but being made of metal rather than plastic, could be quite heavy, and
therefore exuded a feeling of quality!.

Just to add to the other excellent and through answers, golfers (not in
competions) often have a 'rangefinder' device to measure distances to the
pin.

My first few cameras had rangefinders, Contax & Canonet 28 and I liked them.

JH

Benefits:
=========
Smaller and lighter than equivalent metal-bodied SLR.
Ultra quiet (no mirror slap).
Very bright viewfinder.
Extremely fast and accurate focusing, even in low light.
Light and compact lenses, often optically better than SLR equivalents.
Outstanding performance of ultra-wide angle lenses.
Cameras ideally suited to wide angle and standard lenses.

Disbenefits:
============
Non-TTL viewing, so parallax error may be a problem.
No means of viewing depth of field; you have to use DOF scales.
Borderline focusing accuracy with telephotos used wide open.
Difficult framing with telephotos unless accessory viewfinder is used.
Focusing mechanism is delicate and needs periodic checking.**
Lack of automation means operator competence is vital.
Macro work is problematic.

(** except Leica bodies which are very robust and reliable)

A rangefinder camera cannot replace an SLR unless you avoid macro
work, telephoto lenses longer than 90mm (except with a Leica, which
will manage a 135mm lens because of its quality of manufacture), and
can work with depth of field tables.  On the other hand, a rangefinder
camera complements an SLR outfit very well, because each has strengths
where the other has weaknesses.

Hope this helps!

Tony