Cycles is a ray tracing render engine that can be used to achieve photo realistic results in Blender. It was introduced officially in 2.61. Use this tag for any Cycles related posts.
Cycles is a physically based path tracing render engine with GPGPU rendering support, all integrated and bundled with Blender.
To set Cycles as the active render engine, select Cycles Render from the Engine dropdown in the top header:
In Cycles, materials are created via trees of nodes, which are connected by noodles. This allows for complex materials to be created by synthesizing the various types of inputs available, and finally outputting them to a shader, a mathematical equation that describes how light will (or, more accurately, might) bounce.
Rendering in Cycles is accomplished by "firing" and tracing the path of a number of virtual rays of light from the camera through each pixel. Cycles traces the path of each ray, using the shaders to determine how each ray will bounce, and which wavelengths (and thus colors) of light will be reflected. The colors of each ray are then averaged to form the color and brightness for that pixel.
The number of rays fired through each pixel is known as the number of samples. With a low number of samples, a rendered image can look very noisy. This is because there is a large degree of randomness introduced into the bouncing of the rays of light for most shaders. However, as the sample count increases, the randomness stabilizes, and the average color converges to the correct result. The number of samples for a noise-free render varies depending on the scene; it can range from as low as 25, for a scene with just a couple of objects, to a few thousand, for very complex scenes.
Cycles related questions:
Troubleshooting common issues with enabling GPU rendering:
Enabling GPU rendering for Cycles?
How to avoid noisy renders in Cycles?
How to reduce fireflies in Cycles?
How To Reduce Noise In Cycles Render
Spherical emitter does not illuminate very well?
What is multiple importance sample option