PVP Ruleset
When you play on a server of the PVP or PVP RP type, players of the opposing faction may attack you at almost any time, regardless of differences in level or perceived fairness of the encounter. The only areas where you are safe from attacks are areas designated as a sanctuary, such as Shattrath and Dalaran, and areas where your faction's dominance is uncontested, such as faction capitals and the starter areas for each race.
RP Ruleset
RP or PVP RP servers are designed for people who want to role-play in the "World of Warcraft" universe. Because of this, these servers have additional rules governing character names and interactions between players. When creating a character on a RP server, you may not give that character a name that wouldn't fit with the medieval-fantasy theme of the game. Additionally, all interactions occurring in the "say," "yell" and "party" channels must be in character.
Considerations
Although playing on a PVP RP server can be quite interesting, the combination of the PVP and role-play aspects may be confusing for a novice player. Like on normal PVP servers, PVP RP servers have some amount of players who devote themselves to killing players of the opposing faction at all times, especially when the fight is unfairly balanced in their favor due to character level, class or gear. If you want to play "World of Warcraft" together with friends and their characters are located on a PVP RP server, ask for their advice and help in getting used to the various aspects of the game, especially if you keep getting killed by other players.
Alternatives
If you enjoy the role-playing aspect of a PVP RP server but find yourself getting irritated at the PVP combat, consider transferring your characters to a normal RP server or starting afresh on one. Similarly, if you enjoy the PVP aspects but prefer not to role-play, consider switching to a normal PVP server. If you don't enjoy either aspect of a PVP RP server, switch to a PVE server. On this latter type of server, players of the opposite faction may not attack you unless you have marked yourself available for PVP combat, and role-playing is not mandatory.