Recommendation letters deliver a third party point of view to help schools learn more about you, as an individual and within your school community.
There's no way around it: grades and test scores are likely to be the most important part of your college application. But admissions counselors know that statistics don't tell the whole story. You'll write an essay or personal statement for your application, and schools want to learn even more about you — from someone who isn't you. Typically applications will ask for recommendation letters from at least two teachers, but some may allow you to include additional notes from coaches, employers, or counselors.
Like many other parts of the college application process, the trick with recommendation letters is planning ahead. Build relationships with your teachers as early as freshman year, and stay in touch with the ones you like the most. Stay after class to discuss the lesson with your teacher, or ask for help with an essay. That way you'll have a chance to get to know each other outside of the classroom. Make a good first impression, and you'll have an easier time finding someone who can recommend you to your top choice college.
Just because a teacher gave you an A doesn't mean you have to ask him or her for a recommendation letter. In fact, you may want to ask someone who saw you struggle a bit in class, and then overcome that obstacle. Feel free to go to the teacher and ask if he or she feels comfortable writing a recommendation for you. It's your job to make sure the teacher is excited to write to a college on your behalf. You won't actually get to see these letters, so you want to make sure you have a good feeling for what they're going to say.
Remember: you're basically assigning your teachers homework. In addition to giving them plenty of notice, you must also deliver all the materials they need to write the letter. You may need to fill out a form from the college, provide credentials and information for a college's online application system, or get an addressed and stamped envelope. You may also want to give your teacher a list of your activities, goals, and any other topic you think is important so they have ideas for what to write in the letter (don't be offended if a teacher you had in a previous year asks for this info). Above all else, be kind and considerate when interacting with your teacher.