Signs are considered to be a secondary characteristic to spellcasters when compared to their Source. Simply defined, a Sign is the way a spellcaster uses their magic. From conjuring a familiar to creating illusions to unleashing raw, devastating magic — most if not all ways of utilizing magic are categorized into a Sign. Below are the ways that most spellcasting is done — it is noted that there could possibly be other Signs, though no spellcaster has been able to demonstrate it.
Unlike a spellcaster's Source, one could potentially learn and use more than one Sign. However, any Sign beyond their natural-born one tend to be difficult to learn. Because of this, spellcasters who do decide to work beyond their own Sign tend to do it in a role that will support their primary Sign.
It is also noted that not all Sources work with all Signs, for example an Air spellcaster would have issue with being in line of Biothaumaturgy.