First the basics: Presets are Lightroom's way of applying many changes to a photo in one click. They can be made by you for a way in which you commonly treat a photo and would like to apply your own recipe in one click. Alternatively, they can be the recipes of other photographers which you purchase from them as .lrtemplate files to import into your lightroom catalogue of presets. Even better, some photographers give their presets away for free! Keep reading for those.
Photoshop's one-click version are called actions and they work in the same way. Scroll through the effect names, choose one and click. You then see your photograph go through a makeover in a split-second. Again, you can record your own action sequences or buy them as .atn files from other photographers.
I prefer LR presets. I'm a control freak (big time) and I want to have complete, effortless control over every last pixel of my work. I appreciate the foundation that presets can lay in my editing process, but I never click once and move on. I always then play with the sliders to make every last element of my photo exactly what I want it to be. With actions, I feel that I lose control and unless the one click produces exactly what I had in mind (it never does), I don't prefer to use them. The few times I've used actions, they always end up being from Florabella because she makes them work with a variety of layers which you can then tweak. But still, I don't feel that I have enough control.
So what actions/presets do I love? My first love came from the 'Lightroom Killer Tips' website. Matt gives utterly amazing LR tutorials and gives his presets away for free. He even invented a preset extractor program which extracts LR setting from photos on Flickr and that is free as well. Genius! For actions, I use Florabella. She is the love of my textures life and also makes smashing actions. Another place for free actions and presets is CoffeeShop. For presets, my loves are Rebecca Lily and One Willow's 'Retro Candy' presets with delicious names like 'cotton candy'.
Keep in mind that most presets are created to work on RAW files, so if you're not shooting in RAW yet, look into starting that before using presets. If you're not sure about RAW vs JPEG, read this.