So What is an MP3 Encoder?

An MP3 encoder is the software using an MP3 Codec (compression/decompression algorithm), to make MP3s.  Most encoders convert wav to mp3 although many can convert other formats such as WMA to MP3.

There are very few unique encoders.  Most software out there uses only about 4 main encoding engines due largely in part by the patents held by Fraunhofer-Gesellschaft and other companies that helped produce the ISO source that MP3 is based on.  Although no one company owns exclusive rights, MP3 software companies must pay a licensing fee to develop their own ISO source encoder which is expensive.  The major encoding engines are LAME (non-ISO source), BladeEnc, Fraunhofer Encoders, and Xing.