Certificate of Authenticity or C.O.A.
When you buy a piece of my artwork, I send along a certificate of authenticity with it. Many artists use a c.o.a. as a record of information about an artwork and to prove it's authenticity. The certificate contains details such as title, medium, date, signature, contact information, etc., which can possibly make an art buyer more comfortable with buying an artwork. It is a document that an art collector can hold on to for their records, and use as proof of an artwork's genuineness.
Here is a sample of one of mine:
It includes a picture of the painting, the title, medium, size, date produced and the artwork #. It is also personally signed and dated, and has my contact information. I have a copy of each one for my records as well.
I sign my paintings on the front very lightly with my initials, because I don't care to mark it too boldly. It always seems to be distracting to me, so I sign the back.