A resume is typically sent with a cover letter, which is a document that provides additional information on your skills and experience in letter form. A resume is a concise, often bulleted summary, while a cover letter highlights and expands on certain traits or accomplishments that would be unique or ideal assets for the particular job.
Your resume is arguable the single most important part of the application process. A well-organized, relevant resume will set you up to get an interview, while a poor resume will get completely lost in the sea of applicants. This is a fairly modern concern, as job postings now get thousands of online applications a day.