A statement is what is unambiguously asserted by a sentence.
This first defining characteristic of a statement, that it is what is unambiguously asserted by a sentence, roughly identifies a "statement" as what is meant by a sentence, its "meaning." Since two or more sentences in one language, as well as different sentences in different languages, can have the same meaning, the same statement can be expressed by different sentences. Therefore, a"statement" cannot be identified with a "sentence." A "sentence" is in some particular language; a "statement" may be thought of what different sentences which mean the same thing have in common; but a statement is not "in" a particular language. Rather, we say that "sentences" in particular languages may, or may not, make the same "statement".
[For many purposes "statement" and "proposition" may be considered synonymous; however, the two may be distinguished in that a "statement" may be thought of as the meaning of a sentence in a particular context of usage, whereas a "proposition" is abstracted from such a context. Since sentences often contain personal pronouns ("he," "she," "it," etc.) or demonstrative pronouns ("here," "there," "now," "then," etc.) the meaning of which can only be established by context of use, establishing the meaning of a sentence -the "statement"- may require understanding how the terms in the sentence refer to states of affairs in the world.]
A "sentence" is not the same as a "statement"; it is, rather, the vehicle by which the statement is communicated. Thus two different sentences may make the same statement. "Shakespeare wrote Romeo and Juliet." and "Romeo and Juliet was written by Shakespeare," are two ways of communicating the same statement. Moreover, a sentence can be ambiguous allowing more than one equally reasonable interpretation of its meaning; each distinct meaning attributed to an ambiguous sentence is considered a distinct statement.
Remember that sine a statement is defined as what a sentence means, when we talk about a statement we mean that the meanings of the terms in that statement remain fixed. If we change the meaning or redefine any of the terms of the statement, then we are in effect constructing a new statment.