In music, dynamics normally refers to the volume of a composition.
It is relative and do not indicate specific volume levels.

Basics

Notations
Definitions
pianissississississimo pppppp as soft as possible
pianississississimo ppppp extremely soft
pianissississimo pppp  
pianississimo ppp very soft
pianissimo pp  
piano p soft
mezzo piano mp moderately soft
mezzo forte mf moderately loud
forte F loud
fortissimo ff very loud
fortississimo fff extremely loud
fortissississimo ffff as loud as possible

Accented Notes

Notations
Definitions
forzando or forzato fz or ffz begin the note loudly with a very strong accent
sforzando sf or sff or sfff loud and accented with accent mark
sforzato sfz or sffz or sfffz loud and accented with legato

Gradual Transitions

Notations
Definitions
crescendo cresc. or
gradually becoming louder
decrescendo decr. or decresc. or
gradually becoming softer
diminuendo dim. or dimin. or
gradually becoming softer