Compression is an issue that affects both file size and bit rate. Basically, compression allows for file size or bit rate to be reduced. This is accomplished with either lossless or lossy compression.
In lossless compression, file size or bit rate is reduced in size without taking any information away. This typically is able to cut down the file size or bit rate by about half.
Like lossless compression, lossy compression also reduces file size and bit rate. The main difference is that lossy compression actually tosses out some information. Examples of this are JPEG files for photos and MP3 files for music. Since lossy compression actually discards some information, it can achieve much higher levels of file size and bit rate reduction than lossless compression can. However, if too much lossy compression is used, quality will start to become noticeably worse. Technically speaking, the only way to preserve the full quality of digital audio, photo, or video data is to use lossless or no compression, but in many cases this is not practical due to the need to reduce data size.
Given that lossless compression can make things smaller without taking any information away, you might wonder why lossless compression is not used more often. The major downside to using compression - lossless or lossy - is that there might be delays. These delays are also known as lag. For files, it could mean there is an extra delay between when you click an audio or video file and when it starts playing. For bit rate, it can result in a delay between input and output - seeing a delay between moving your mouse and when the cursor moves on the screen is an example of this. Note that compression does not necessarily mean that lag will occur, but compression does make lag a possibility.