k-lite Codec Pack Mega/Full/Standard

Standard
Media Player Classic Home Cinema (MPC-HC)
This is an excellent player. Highly recommended!
It provides high quality playback and many useful options.
MediaInfo Lite
This is a tool for getting details about media files.
This is the recommended variant for the average user. Use this if you don’t know what you need. It already contains everything that you need for playback. The extra components that are included in the larger versions provide no benefit for the majority of users.

Full

Same as Standard, plus:

MadVR
An advanced video renderer with high quality upscaling algorithms.
Plugin for 3D video decoding (H.264 MVC)
Note: this requires using MPC-HC with madVR, and also a compatible graphics driver. Recent NVIDIA drivers no longer support 3D video (but you could try “3D Fix Manager”).

Mega

Same as Full, plus:

DC-Bass Source Mod
For decoding OptimFrog and Tracker audio files (very rare formats).
GraphStudioNext
A tool for creating and testing DirectShow graphs.
A few ACM/VFW codecs such as x264VFW and Lagarith
This type of codec is used by certain video editing/encoding applications for working with AVI files. For example VirtualDub.
These type of codecs are not used or needed for video playback!
ffdshow audio processor
DirectShow filter that provides some audio processing options. Not used by default. Also not needed or recommened.
ffdshow video processor
DirectShow filter that provides some video processing options. Not used by default. Also not needed or recommened.
Important note:
The K-Lite Codec Pack does not expand the import abilities of professional video editors such as Vegas Movie Studio or Adobe Premiere. Those applications often only support importing a small set of file formats, and do not support using the type of codecs that are included in the codec pack (DirectShow/VFW). Modern editors often only use their own internal codecs or only support external codecs of the Media Foundation type.