In Guitar playing, the fingers on the right-hand (the picking hand) have specific names. The letters that represent the Thumb, index, middle, and ring fingers are: P-I-M-A.
These letters are abbreviations of Spanish words, "P" = PULGAR
Pulgar = Thumb. "P", The thumb, is mostly used to play bass notes. Moving in a down motion only. Some flamenco players and some bass players train using an up-down motion with the thumb.
The next Spanish word is Indice = Index Finger = "I".
Because most guitarist are best playing with "I" instead of "M" or "A", many guitarist play the melodies of songs with "I" to make them more pronounced.
The next Spanish word is Medio = Middle Finger = "M".
The middle finger is not typically used by itself. M usually is played in the context of a pattern (such as P-A-M-I). It can be very useful to practice playing loud and clean with "M" in case you ever need it for strong melodic or accented note.
The last Spanish word is Anular = Ring Finger = "A". The ring finger, while being used in plenty of music, is used the least of the four digits. To develop its strength I recommend playing scale passages using "M-A" instead of "I-M". Remember, you will need a confident "A" when trying to make a beautiful tremolo.