Normal wear and tear and damage are two very different things when it comes to renting. With one, you are still entitled to your security deposit, with the other, the landlord can keep some or all of your deposit. Rental Housing On Line helps you differentiate between the two.
A landlord can use your security deposit to repair damage that you are responsible for, but they cannot use the security deposit to pay for normal wear and tear, which is disrepair that is the result of normal conditions. No matter how clean or careful a tenant is with an apartment, paint is going to fade, electrical switches may stop working, strings on blinds can break, carpet and tile need replacement, etc.
Damage happens from an accident or negligence on the part of the tenant. It can be an accumulation of dirt and mold beyond a reasonable amount, stained carpets, and windows that are broken. You also can have your security deposit docked if you leave large holes in the walls from hanging things on them, or repainting the walls without written permission of your landlord.
Both the landlord and the tenant should, and in some states are required to, take certain steps to ensure disputes over damages do not occur. When a tenant first moves into a property, they should inspect everything thoroughly and note any problems on an inspection check list, which should then be signed and dated by both parties. When the lease term is up, the tenant should again go through the apartment with the landlord and talk about any damage, checking any problems that are found against the move-in check list.