Examples of what jobs the different sizes are typically used for are:
A0/A1/A2/A3 – Posters
A4 – Letterhead/Magazines/Reports
A5 – Magazines/Flyers
A6 – Invites/Flyers/Postcards
Another commonly used size is DL which is 99mm x 210mm which is 1/3^rd the size of an A4.
Business cards are typically 85mm x 54mm