All spellcasting classes have a spell list. This is essentially a list of what spells that class can cast and at what spell level they become available.

The lists are usually based around the class concept.

e.g.
Druids are casters who revere nature so most of their spells focus mainly on producing natural phenomenon.

Both clerics and druids have a healing focus so they both gain access to the cure moderate wounds spell, but because clerics are more inclined towards healing, they recieve this spell at level 2, while druids recieve it at level 3.