Rear Output and Input Connectors allow different output devices to be attached to a receiver. Receivers will vary in the quantity and type of rear inputs and outputs offered. Here are definitions of the most common ones:

Composite (Video)
A composite video connector uses a standard RCA-style connection that passes a combination of chrominance and luminance information on a single cable. This connector might be used to connect a DVD Player, VCR or camcorder. Though capable of delivering a high-quality picture, a composite video connector is not considered as accurate as either S-Video or Component video connectors.
RCA (2 Audio Channels)
RCA is an audio connector consisting of left and right stereo channels and is used with most audio components with analog outputs.
Digital Coaxial
Digital Coaxial is a type of connector that allows a receiver to accept digital signals, which produce higher-quality results than analog signals. For example, a Digital Coaxial connector could accept Dolby Digital and DTS signals from a DVD player. Digital Coaxial connectors use modulated radio frequencies (RF) and require a special cable. Another type of digital connector is Digital Optical.
Digital Optical
Digital Optical is a type of connector that allows a receiver to accept digital signals, which produce higher-quality results than analog signals. Digital Optical is the most common digital connector, and transmits data using laser pulses over a fiber optic cable (Toslink) that connects with a standard RCA-style jack. A Digital Optical connector might be used to connect with a DVD Player or a CD player. Another type of digital connector is Digital Coaxial.
Component Video
Component Video connectors use three RCA jacks. One carries the brightness (luminance) portion of the video signal, and the other two carry the color (chrominance) portion. As component video connections split chrominance between two jacks/cables, they provide higher-quality video transfer than an S-video connection. These connectors are commonly available on DVD Players and higher-end televisions.
S-Video
S-Video connector use a 4-pin connector with an S-video jack that transmits the chrominance (color) and luminance (brightness) portions of a video signal separately. This format reduces distortion and improves color accuracy, providing higher quality video than Composite Video, but lower quality than Component Video. These connectors are commonly used to connect to digital sources, such as DVD Players, S-VHS VCRs, and digital camcorders.