Usually it is the process by which the musicians learn to identify rhythms, intervals, chords and other essential elements of music. Some people have what is called perfect pitch or absolute pitch, they normally when listen to music can tell exactly what they are listening.
Nevertheless one does not need absolute pitch to succeed at to ear training, the main of ear training is the development of relative pitch.
In other words, if you play 2 notes, they can tell you that one of them is a major third higher than the other, and also they can tell what your you will play.
Luckily, having relative pitch is really very good and for several musicians it can be more useful than perfect pitch.
Functional pitch recognitionIt is an important skill for the musicians who usually listen and create music, generally involves to recognize the function or role of single pitch in the relation of an established tonic.
Several musicians use the Functional pitch recognition to understand, identify and value the roles and significant of pitches within a key.
Rhythm recognitionFor example, one might start by studying the sound of all the combinations of 4 - 8 notes and 8 rests, and then proceed to string different 4-note patterns together.
Interval recognitionIt's also a useful skill for all the musicians, to be able to determine notes in a melody, usually a musician must have the capacity to recognize intervals.
Timbre recognitionGenerally each musical instrument has a quality of sound and some instruments have more of a timbre, a basic example the plucked violin has a sound different from the sound of a bowed violin.
Learning to recognize and to differentiate several timbres is a very important musical skill that it is possible to be obtained and to be improved with the training.
Chord recognitionGenerally to recognize melody of a song is listening to the harmonic structures that accompany, it is for that reason than the musicians practice to listen different types of chords.