IV just stands for IntraVenous, so almost any fluid can be given that way, but the standard solution that is given is saline to which you can add whatever additive you need.
Typical saline solution just contains sterile water and sodium chloride (salt). The concentration of the salt can vary depending on what it is needed for. Two of the more common concentrations are 0.9% (normal saline) which is isotonic to blood, and 0.45% (also called 1/2 normal saline), which is hypotonic to blood.
Another common additive to saline is dextrose, which like saline comes in carrying concentrations.
There are also some specialty IV solutions such as Ringers Lactate and Acitated Ringers, that are used in certain circumstances.