"Treaty means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation."
This, from Article 1 of the 1969 Vienna Convention on the Law of Treaties.
According to the Canadian C.E.D. treatise on International Law (2011):
"It makes no difference whether the agreement is called a treaty, convention, charter, covenant, statute, or any other name so long as it is clear that the intention is to effect an agreement."
In Trans World Airlines, Madam Justice Sandra Day O'Connor of the Supreme Court of United States wrote:
"A treaty is in the nature of a contract between nations."
The Berne Convention is an example of a treaty.
Treaties are just contracts between a select number of states by which the signatories to conduct themselves in a certain way or to do a certain thing, such as the North American Free-Trade Agreement (NAFTA).
These latter type of treaties are usually private to two or a limited number of states and may be binding only through the International Court of Justice.
A treaty may be law-making in that it is the declared intention of the signatories to make or amend their internal laws to give effect to the treaty. A treaty is of no force or effect anywhere except within and binding upon those jurisdictions that have formally adhered to it, usually by domestic statute that refers or incorporates it.
Another important distinction between a treaty and a conventional contract is that a treaty lacks any enforcement teeth. A conventional contract will readily be enforced by a Court of law subsequent to which the Court will issue an order, usually for damages. This is enforceable up to an including against the assets of the party that breached the contract, all the while with the enforcement provisions of the Court and the not-insignificant assistance of the Government and law enforcement powers in regards to the enforcement of judgment. This power - and the culture of compliance with one's contracts it maintains - keeps most people and corporations to their contracts. In the international environment, there is no international law enforcement body so at the end of the day, there is no consequence to a state breaking a treaty except a loss of international credibility.