Affidavit and statutory declaration are mostly used for a similar purpose. Although these terms are often used interchangeably, they are not entirely the same.
An affidavit is a written legal statement of fact set out by someone who has personal knowledge of them. It is confirmed by oath or affirmation for use as evidence in court proceedings.
A statutory Declaration refers to a written statement of facts, sworn to and signed by a deponent before a notary public or some other authority having the power to witness an oath.
The major difference between these two has to do with where it is used. An affidavit is used in court proceedings while the statutory declaration is used outside of court proceedings.