A declarative sentence is a sentence that asserts a truth or falsehood, and is either true or false.
"The moon is round" is a declarative sentence.
Other sentences can be
interrogative, asking a question: “Is the moon a balloon?”
exclamatory: “There goes that green girl!”
or imperative, expressing the need to do something: “Drop the bomb on Washington”.
Logic is full of declarative sentences that must be either proved or disproved if possible. Not all declarative sentences can be proved or disproved. “It is right” cannot be proved or disproved since it is not clear what is being referred to. “Isosceles triangles have two equal sides” can be proved. If a declarative sentence can be proved or disproved it is called a declarative statement. “It is right” is not a declarative statement but “Isosceles triangles have two equal sides” is a declarative statement.
Every declarative statement has a truth value (1 or T if it is true and 0 or F if it is false). Sentences can be combined into arguments and truth values manipulated using Boolean Logic or truth tables to prove or disprove entire arguments on a rigorous manner.