What exactly is a waveguide?

A material which can guide wave.

Waveguide is a physical structure which guides electromagnetic waves. Waveguide size and its material depend on the frequency. Waveguides can be constructed from either conductive or dielectric materials.

CMIIW

PS: For more information about waveguide, you can read any basic electromagnetic topic books...

Br,

Hi sumant.thapliyal,

The two-wire transmission line used in conventional circuits is inefficient for
transferring electromagnetic energy at microwave frequencies. At these
frequencies, energy escapes by radiation because the fields are not confined in
all directions.

Waveguides are the most efficient way to transfer electromagnetic energy.
WAVEGUIDES are essentially coaxial lines without center conductors. They
are constructed from conductive material and may be rectangular, circular, or
elliptical in shape.

I guess 2 wire transmission line is also a waveguide provided it excites a single mode on it.
And at higher (microwave) frequencies it is skin effect phenomenon casuing loss of energy in coaxials...so we switch to hollow waveguides...
Is is what Balaguru you are saying...? Am I right...?

A waveguide is basically something that guides a wave. When a wave is received from outer space, or some other source, we need to send it in a particular direction to our circuit or other region of interest for further study. Using the boundary conditions of Electric and magnetic fields, the waveguide can be designed to break up the received wave into particular modes and polarizations for further study...

See David Cheng, "ElectroMagnetics," or Simon Ramo, "Wireless Communication" for a more mathematical derivation. If you are really feeling good about yourself, look up "Electrodynamic" by Jackson.

it's use is somewhat similar to a coax but for RF signal.