They obviously match the diameter and threading of the screws, and look like the following

Thinner screw for lighter loads (5mm outside diameter, .8mm pitch)

The longer ones are generally 20mm, and shorter is 16mm.  This will be denoted as M5x16 or M5(16) vs. M5x20 or M5(20). 

Thicker screw for heavier loads (6mm outside diameter, .8mm pitch)

We still use them in the states, but it's probably the best plan to stop getting these, and only focus on the metric screws.  

Standard imperial screw, .19" outside diameter, and 0.031250" pitch

Less used, thicker imperial screw.  Outside diameter is 0.2160", and the pitch is 0.041667".