To stay clear of any legal involvement, both parties concerned must comply and honor all agreements made. Since the loan agreement serves as a contract and legal documentation, a violation of the terms and agreements can lead to a legal case and the written agreement can function as proof in court.
A loan agreement is a contract between a borrower and a lender which regulates the mutual promises made by each party. There are many types of loan agreements, including "facilities agreements," "revolvers," "term loans," "working capital loans." Loan agreements are documented via a compilation of the various mutual promises made by the involved parties.
There are many financial institutions that offer private student loans with bad credit. The only fuss in the deal is the requirements of the financial institutions. Most entities that I know of providing this service would require a Co-borrower or a Co-Signer. This means as a student you and your parents would need to sign on the loan agreement agreeing to repay the loan.
For commercial banks and large finance companies, "loan agreements" are usually not categorized although "loan portfolios" are often broadly characterized into "personal" and "commercial" loans while the "commercial" category is then subdivided into "industrial" and "commercial real estate" loans. "Industrial" loans are those that depend on the cashflow and creditworthiness of the company and the widgets or service that it sells. "Commercial real estate" loans are those that repay loans but that depend on the rental revenues paid by tenants who lease space, usually for extended times. More granular categorizations of loan portfolios exist but these are always variations around the larger themes.