Actually, feomateo is very wrong. It is not at all about which one sounds cooler. All three are very necessary.
SLA - Service Level Agreement
Defines the level of service the provider will offer. Expressed in xx.xx% form, usually includes clauses concerning acts of God, hardware failure, etc, etc...
AUP - Acceptable Use Policy
Dictates how the service can be used; for instance, whether or not adult content can be hosted
TOS - Terms of Service
This is the meat of the relationship between host and client. It defines the services provided, the roles played by the host and the client, and the responsibilities of each.
Daniel B., CEO - Bezoka.com and Ungigs.com Hosting Solutions Optimized for: WordPress • Joomla • OpenCart • Moodle Data Centers in: Chicago (US), London (UK), Sydney (AU), Sofia (BG), Pori (FI) Email Daniel directly: ceo [at] bezoka.com
SLA: Service Level Agreement
An agreement between the client and the provider for a certain standard of service. Service levels can be defined in many ways and are dictated by the provider. Typically outlined in an SLA are: Network uptime, Server uptime, resource availability and support response/resolution times.
TOS: Terms of Service
The terms under which you must operate to exist as a client of the provider. This is commonly teamed with the acceptable use (or usage) policy
AUP: Acceptable Use/Usage Policy
This is a contract between provider and client which outlines what is/is not acceptable use of the service being rendered. This usually outlines what is defined as abuse of network, resources, etc.
These contracts shouldn't be taken lightly. They are not there purely to "look cool" or "look professional". These are all legally binding contracts that keep both parties in check. I'd advise you consult a lawyer in your area in order to have contracts drawn up that would be legally binding in your area.