A Necromancer controls a pwerful pet. They get 3 types of pets which are generally classed as tank, scout, and mage. The particluar pets fill roles and have strengths/weaknesses as their class indicates:
Tank: lots of HP and defense. Low DPS.
Scout: medium HP, defense, and DPS.
Mage: low defense and HP, high DPS.
Each pet can be used to match the situation. If you are soloing tough mobs, then use the tank pet. If you are in a group with a stsrong tank and healer, use the scout or mage pet.
Necro's also have life tap ability to transfer life between him and the pet. At higher levels they get a Resurrect and Escape (evac).
Warlocks are the evil version of Wizards. Both are masters of high damage. Basic tactic is root/nuke/repeat.
Both classes have buffs to assist the party, but I feel a Necro would be a more challenging class to play. Bear in mind that there is a significant Live Update coming to change the way Summoners (Necromancer/Conjurer) are, basically making life tougher for them.
ALl classes have a viable place in a party. I've even been in all caster parties with no tank or healer, and we did great!
Most important, play one that is fun for you, as that will make the game experience that much sweeter. :)