Dynamics

Dynamics is a generic word meaning indications of volume.
A composer uses them to indicate to the performer(s) the volume of a piece or part of a piece of music.
As with other musical directions (such as speed indications), they are usually written in Italian, which is the internationally used language for music.
These indications are written as abbreviations near the stave, but not on it.
These abbreviations are written in a curly, bold italic font.

All the common abbreviations are for Italian words.

Italian word(s)	Literal translation	English meaning	Abbreviation used
piano	soft	soft	p
forte	loud	loud	f
mezzo-forte	half loud	moderately loud	mf
mezzo-piano	half soft	moderately soft	mp
pianissimo	as soft as possible	very soft	pp
fortissimo	as loud as possible	very loud	ff
forte piano	loud soft	loud then immediately soft i.e. an accent	fp
forzato or sforzato or sforzando	forced/forcing	accent	fz or sf or sfz
rinforzando	reinforcing	sudden accent	rf or rfz or rinf
crescendo	get louder	get louder	cresc. or
diminuendo or decrescendo	get softer	get softer	dim. or *decresc.* or

Composers do use other abbreviations such fff and ppp which have meanings which are obvious extensions to the above, but they are not actually abbreviations for anything!
The visual signs for crescendo and diminuendo are printed as long or short as necessary to indicate how long the getting louder or softer should last. They tend to be known as "hairpins", for hopefully obvious reasons (as in a pin to hold hair, also used in the phrase "hairpin bend"). They are usually written above or below the stave.