Homes wear out even when tenants are careful. Faded paint, light scuffs, minor carpet wear in walkways, small nail holes from pictures and loose door handles are typical examples of normal wear and tear.
Damage is different: harm beyond ordinary use, caused by accident, neglect or misuse. Large holes in walls, burns or pet stains in carpet, broken windows and unapproved paint colors are typical examples. Damage can usually be deducted from the deposit; wear and tear usually can't.
The line isn't always obvious, and how long the tenant lived there matters: carpet that is worn after eight years is wear, while the same carpet ruined in six months may be damage. Dated move-in and move-out photos are what settle the question.
Free resource: Move-in / move-out inspection checklist.