As I was leaving college, I had a class that taught me about putting together our resume and taught us how to go about getting through an interview, and how to make a good, lasting impression. One of the most important things our teacher stressed was having a letter of recommendation from at least three people. Though many times people have references, nothing speaks more highly of you than a good letter written by someone you have either work for or who is known you for a long time.
The best letter of recommendation will come from a former employer. Though this is not always possible, if you can get one of these, you have put yourself in a better position. This type of letter of recommendation will show that you head a good relationship with your prior employer. Many people leave jobs on bad terms, and when this happens, employers may become leery of hiring you. As you may understand, they want people within their company who are easy to work with, and who will be willing to leave the job under good terms if and when the relationship ends.
As with any career, there may be many stops along the way before you reach your dream job. As you move up the ladder and switch companies, you will want to ask for a letter of recommendation from those you have worked for. You may even ask for letters from your coworkers if they have worked with you closely on any number of important projects, and have enough clout to make a difference. A letter of recommendation from the company mail person will not be as effective as a letter from the CEO. However, you have to work with what you can get. Sometimes a good letter of recommendation from someone low on the ladder is better than none at all.
You will not want to give copies of a letter of recommendation to an employer until they ask for one. This is something you want to note at the end of your resume. You may say that references are available upon request, or that letters of recommendation are available. Do whatever you think is best. Be sure that you have your letter of recommendation with you when you go in for an interview. This is when they will most likely ask to see it. If they do not ask you, do not force it on them. That will make things awkward for you and for your potential employer.