5-40 X 1/2"

It's a #5 screw (1/8" diameter) with 40 threads per inch and is 1/2" long.

Machine screw diameters go from zero to 12 in size, and then are normally in inches after that. See the chart in the link below:

http://en.wikipedia.org/wiki/Unified_Thread_Standard


M2.2x4.8mm

This one is sort of misleading as it leaves out the thread size, but it is a 2.2 mm diameter screw that is 4.8 mm long (and presumably a fine thread of 0.25mm between threads as they don't make a coarse thread machine screw with a 2.2mm diameter)

Typically, one might see a M4 - 0.7 x 6

This would mean a 4 mm diameter screw with threads spaced 0.7 mm apart and a length of 6mm

See fine and coarse metric screw charts in link below:

http://www.engineeringtoolbox.com/me...ads-d_777.html

Notes:

In general, machine screw length is from under the head to the end of the threads except for countersunk screws where it's the total length from the top of the head to the end of the threads.

Note English screws use threads per inch, but metric use mm between threads.