RADAR Sensors have a transmitter which send a pulse of electromagnetic radiation through the antenna at a specific wavelength and duration. These are off-nadir imaging sensors meaning that the have a look direction and angle which is controllable.
The microwaves transmitted by the RADAR Sensors are much longer than visible, infrared or thermal infrared energy so common to passive systems (Landsat, WorldView, Pleides, Quickbird). Microwaves are measured in centimetres not micrometres.
If you see Radar associated names with letters like X, C, S, L, these are references to the wavelength of the signal.
For reference, the C wavelength is 6cm and the L wavelength is 24cm.
C is a the generally the wavelength used unless you have a specific goal (i.e if you are trying to see through the atmosphere a longer wavelength can be more appropriate). K band is useful for topographic relief.
Often time when you hear about a RADAR system you will hear it referred to as SAR (Synthetic Aperture RADAR). SAR uses electronic means to artificially increase the length of the antenna which improves resolution using smaller sensors.
You may also hear mention of SLAR. SLAR stands for Side Looking Aperture Radar. SLAR is much larger as it does not artificially increase image quality.
It is beneficial to fly RADAR surveys in only one direction so that the illumination is always the same. Look direction can affect which types of features are best imaged. Linear features that that are perpendicular to the look direction of the instrument will have an enhanced response in comparison to parallel features. This can also be affected by the polarization of the microwave (can be horizontal or vertical). Often RADAR sensors will transmit and receive either horizontal or vertical waves (or both).
Radar imagery is displayed in slant-range geometry, meaning that it is displayed based on the actual distance from the radar to each of the features in the scene. This can be converted into ground-range display so that features are in a proper planimetric (x,y) position relative to eachother (instead of to the sensor). It could help to think of a police radar gun. It tells how far it is from the gun and with a bit of math how far a car is moving. It does not tell how far one car is from another. By calculating the distance from the radar sensor to each car you can determine their distance from each other.
Resolution of an image is dependent on range and azimuth resolutions of the radar. The shorter the pulse, the finer the resolution.
Jensen, John R. (2006) Remote Sensing of the Environment: An Earth Resource Perspective, New Jersey, Prentice Hall.