How do letters of recommendation work?

<p>I'm going to be moving my senior year, so I'd like to ask for letters of recommendation from my teachers now. Do they need to know what schools I'm going to apply to? Should I ask for sealed, unaddressed copies that are not addressed to a specific college? How many copies should I ask for?</p>

<p>Thank you. Any additional information about the whole process is appreciated. (:</p>

<p>Also, anyone know how many teachers submit them online? Just because I'm wondering if that would be easier, it looks much easier. I'd have to wait until August to ask them, and just hope that they remember me I guess, but I don't want to be a bother...</p>