No matter what your career field, you will likely need to prepare a resume, cover letter and fill out an application when looking for a new job. The main purpose of your resume is to provide the company with an overview of your professional achievements and background, while an application will tell them the more specific information they need as it pertains to the job.
By submitting a resume and application, you are conveying to the employer your intent to apply for the job in question. The cover letter that accompanies these materials is often referred to as a "letter of intent" for that reason. Virginia Tech Career Services also points out that your resume's purpose is not to get you a job, but rather an interview. The resume and application are merely the first step in the process, signaling your intentions.
Despite the hours of time you may put into preparing your resume, an employer may only spend a few seconds glancing at it before deciding whether or not to contact you. Therefore, another purpose of your resume is to give the employer all of the most important information about your career, organized in a concise manner so that a quick scan will tell him most of what he needs to know. Likewise, a company's application will contain specific questions and fields that you should answer briefly so they can instantly get an idea of the type of employee you would be.
After you're hired, the company will keep your resume and application on file. It's important to be completely honest on both, because something may come up later where a statement you made on one of those documents in regards to your experience or skills may be held against you. In some cases, your application may also spell out your job requirements. As with any document you sign, you must read it carefully and consider the information you input as a potential employee.
Your resume in particular provides all of the details of your career up to this point. This includes your education and all degrees and certifications, all of the jobs you have held and currently hold and a list of skills you developed as a part of those experiences. The application may also ask you for specific information regarding your career path thus far, such as whether you've worked abroad or have a particular skill necessary for the job that isn't spelled out in the job description.
Both the resume and application also serve the purpose of showing the employer your intent for the future. The "Objective" on your resume is typically the first section and consists of a statement in which you declare your career goal that begins with applying for this job. The application will likely also ask you to answer questions that will help the employer get an idea of how you will fulfill the position.