Welcome back to the game then.
Templates are essentially themed modifications that can be made to monsters and/or characters. Sometimes they only apply to one specific type of creatures (like only humanoids) or they can be applied to virtually any creature (like giants, humanoids, dragons, etc).
Templates in the SRD (or MM) include (but are not limited to): Vampires, Liches, Ghosts, Lycanthropes, Fiends, Half-Fiends, Celestials, and Half-Celestials.
Templates are tools for the DM to make an encounter more interesting. Each template includes what kind of creatures they can be added to and what modifications they make to the creature.
For example, say you wanted to make a Fiendish Wolf for the party to fight against. You'd take the statistics for the wolf and apply all the modifications that the Fiendish Template gives (the wolf type changes from animal to magical beast, it gets darkvision and energy resistances, limited SR and it's intelligence goes to 3. It can also Smite Good 1/day). The template also lists how it modifies the CR. In the case of the wolf, it doesn't modify the CR at all.
Add the template to a troll however and it modifies the CR by +1 and you have a really interesting encounter. A troll with Fire Resistance 5 and Damage Reduction 5/magic becomes a little bit tougher.
There are a lot of templates out there, including many undead-based and elemental based templates - even a half-golem template.