What is the difference between a treaty, accord, convention, a declaration or a pact?

Many different terms may be used to describe agreements in international law. According to Article 1,1 of the UN regulations regarding the registration and publication of treaties and international agreements under Article 102 of the UN Charter, the nature of the international agreement is important, not the descriptive name (emphasis added):

1. Every treaty or international agreement, whatever its form and descriptive name, entered into by one or more Members of the United Nations after 24 October 1945, the date of the coming into force of the Charter, shall as soon as possible be registered with the Secretariat in accordance with these regulations.

The Vienna Convention on the Law of Treaties (VCLT) defines several common treaty terms, including the term "treaty": 

Article 2,1(a) states:

"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;

The Treaty Handbook, section 5.2, also offers some guidance on what is considered a treaty under Article 102 of the Charter. 

The use of the word "declaration" indicates some level of solemnity and may be used in several ways: 

• Some UN Resolutions of a solemn nature are called declarations, examples include: the Universal Declaration on Human Rights (A/RES/217(III)[A]) or the United Nations Declaration on the Rights of Indigenous Peoples (A/RES/61/295)
• Political declarations made by a group of countries or regional organization may be transmitted as UN documents through a letter or note, for examples of these types of political declarations, see these search results.
• A specific type of treaty action a state might take in participating in treaties is also called a declaration. For more information on these types of declarations, see the UN Treaty Handbook (Section 3.6) or a scholarly text on the law of treaties. 

Further guidance on the nature of treaties, declarations and related terms can be found by consulting scholarly texts on international law. For suggestions on which texts to consult, see the Peace Palace Library's research guides on diplomacy, international relations, and treaties. Each research guide will have a useful and extensive bibliography of suggested readings on the topic.

You can also find lectures on the Law of Treaties and a Research Library on treaties, including links to relevant treaty series, in the UN's Audiovisual Library of International Law.