LAME Ain't An MP3 Encoder (LAME) is an encoder that converts and compresses any input audio file and outputs an MP3 file. The resulting MP3 file can have a constant, variable, or average bitrate. RED recommends LAME as an MP3 encoder because it is open source, customizable, and outputs high quality MP3 files.
When encoding a constant bitrate (CBR) file, the user (you) chooses a preset bitrate and LAME targets that bitrate throughout the entire file. This means that every second in the file has the same number of bits, no matter how simple or complex the sound is. Every second in a CBR file has the same quality. Because silence is given the same number of bits as more complex sounds, CBR files are larger than VBR and ABR files of the same quality. This also means that CBR files have a predictable file size.
When encoding a variable bitrate (VBR) file, the user (you) chooses a preset quality and LAME targets that quality, letting the bitrate vary throughout the entire file. This means that every second of the file has a different number of bits that depends on how complex the sound is at that second. For example, a second of silence would receive much fewer bits than a second of loud, blaring music. Every second can range from 0 to 320kbps (the ceiling for MP3) based on the preset and the audio data being encoded. Since VBR files target a certain quality instead of a certain bitrate, exact VBR file sizes are more unpredictable. The resulting filesize for a VBR encode depends on two factors: the preset selected, and the music itself. The better the preset, the larger the resulting file will be. The more music data necessary to encode, the larger the resulting file will be. When one refers to the 'bitrate' of a VBR file, they are talking about the mean bitrate (in kilobits per second) for the whole song, which is calculated by dividing the filesize by the duration.
LAME has certain VBR presets V0 to V9. V0 is the highest quality VBR preset and V9 is the lowest quality VBR preset. The most popular VBR preset on RED is V0, which usually ends up with a bitrate between 230 and 270 kbps. V2 is the lowest quality LAME VBR preset allowed on RED for music torrents, and generally results in a bitrate between 180 and 210 kbps. At RED, MP3's encoded with the V2 preset are trumpable by those using the V0 preset.
When encoding an average bitrate (ABR) file, the user (you) chooses a preset bitrate and LAME allows the bitrate to vary throughout the entire file, but the average bitrate of the file will be the bitrate you preset. This means that like CBR, the file size is predictable, and like VBR, the quality and bitrate of the music varies throughout the file depending on how complex the music is each second.
However, ABR is not recommended by RED because it is a mix of both CBR and VBR, and it is thus unable to perform well on either. (Jack of all trades but master of none.)