The Artbox is the smallest box within a PDF file. It covers the area to be used when importing the PDF file into another application. It is comparable to the size specified during the import of an EPS file.
The Trimbox is the final format of a PDF file without bleed. For an A4, the Trimbox of the PDF is therefore 210x297mm, while the Bleedbox would be 216x303mm with 3mm bleed all around.