The etymological origin of the word credit goes back to the word latin credĭtum, who wants to say "entrusted thing." That said, the concept of credit has to do with trust.
A credit is a sum of money which is due to an entity (e.g., a Bank) or a person. An entity that grants a credit of 10,000 euros to a client is trying to lend him this amount; This person will have him make money within a given period, generally with the interests to which the Bank has the right (credits are profitable).
Moreover, the creditor (the one that grants the credit) has the right to require that the loan is repaid. If ever the person fails of its obligation to pay the credit, it may suffer legal consequences (by the Court, for example).
In another context, the term credit is used as a synonym of authority, trust, reputation or esteem: "fans are always credit team to reverse the situation.
In the area of the law, a letter of credit (or letter of credence) is a letter that an Ambassador or Minister shall submit to the president to which he is accredited, attesting to the quality of the Envoy.
Finally, the credits are compulsory mentions of the name of the owners of the rights of reproduction of photographs illustrating a book (in printing) or a web page.