Trying for a longer answer than the others.
Art of War is a set of miniature wargame rules. It is intended to be played with scale miniatures representing the various troops of antiquity. The game can be played with just card bases and no figures but it looses a lot of the visual appeal of a wargame and this would not be allowed in a competitive arena, as a means to learn the game though it is fine.
Miniatures are available in various sizes (essentially height in mm) and there are dozens of manufacturers to choose from. Most of these figures are cast in lead alloys and sold unpainted
The largest comonly used size is 28mm (the same size as Games Workshop fantasy figures),
The next smaller scale is 25mm, this used to be the most common scale and most 25mm figures are older models
In the last 5 or so years there has been a huge growth in the availability of 20mm plastic figures. These are like the old Airfix or Revell figures you may remember but there are hundreds of different set available (
www.plasticsoldierreview.com) lists and reviews all the known sets.
The most popular scale is 15mm and there are thousands of different figures to choose from. Major manufacturers include Essex miniatures, Old Glory and Xyston.
If you want smaller figures there are companies that make 10mm, 6mm and even 2mm miniatures. Massed smaller figures on the same base sizes as 15 or 25mm figures look very good.
Price wise the cheapest are the 20mm plastics, then 15mm and 25/28mm.
If you are looking to get into miniature wargaming there are often people selling second hand painted figures either on ebay or at bring and buys sales at wargames shows.
An army will be made up of perhaps 2-300 figures. Some players have just one army, other (like me) have dozens...
Hope that helps
Hammy