A ‘declaration' is generally a non-binding written instrument setting out international principles. It may be of political or moral force. Declarations are commonly drafted and approved by international organs and conferences, but they are not signed and ratified by states.

A ‘treaty' is a legally-binding international agreement concluded between States in written form and governed by public international law, whatever its particular designation.

Such instruments can be called ‘treaty', ‘convention', ‘covenant', ‘agreement', ‘protocol' or some similar word.

Treaties can be bilateral, multilateral or plurilateral.

Optional Protocol:
Very often, human rights treaties are followed by ‘Optional Protocols' which may either provide for procedures with regard to the main treaty (for example, creating an individual complaint process) or address a substantive area related to the treaty. Optional Protocols to human rights treaties are treaties in their own right, and are usually open to signature, accession or ratification by states that  are party to the main treaty.
A ‘reservation' is a statement made by a state when ratifying or acceding to a treaty which excludes or modifies the legal effect of a particular treaty provision as it applies to that state. Reservations have a binding effect on the treaty monitoring body and on other states that are party to the treaty vis-à-vis the reserving state.
Interpretative declaration:
An ‘interpretive declaration' is a statement made by a state when ratifying or acceding to a treaty, which specifies the interpretation a state will give a particular treaty provision. Unlike reservations, these statements do not exclude or modify the effect of the treaty, and they are not binding on the treaty monitoring body or other states that are party to the treaty, although they are intended to influence the subsequent interpretation of the provision.