I could probably do a ten part series on fasteners, types, plating, torque, uses, cutting threads, assembly, washers, thread lockers etc. But, I'm going to keep it simple. If you want to know more, look it up.
In the rest of the civilized world metric fasteners are used primarily and referred to as SI or System International.
In the US, metric fasteners are used as well as SAE or Society of Automotive Engineers.
When you buy a bolt, say an SAE 3 inch long bolt, 3/8" in diameter and UNC or UNF (coarse or fine thread), you would ask for a 3/8 UNC (16 threads per inch or TPI) x 3" long bolt. A metric bolt approximately the same would be M (metric) 10mm 1.0(1 thread per mm) X 75mm long. Make sure you get the right bolt and the correct pitch, ie # of threads per inch or mm. Your hardware guy should have a little "pitch gauge" to match you up. The head of the bolt will have either an M (metric) on top or slash marks (SAE) (see chart below) If you mix up M and SAE bolts, things will not go well for you. Never over torque your fastener. If you do, you may damage or break it.
Also, when you buy bolts they are graded ie grade 5 SAE or 8.8 metric. These are the strength ratings, how much you can torque them. There are torque charts for each size and type of thread. SAE go from 1-9, metric up to 10.9.
There are exceptions for special bolts. When you buy bolts, the nuts and washers must match by strength. That's right Bubba, you can't use a washer from your deck to back up your suspension bolts. As a rule, don't use lock washers. (They fail, like Skylights) Use a flat washer and a thread locker like Loctite.
Two technical terms to understand:
Tensile strength: If you pull on a bolt(stretch it) till it breaks you just tested the tensile strength.
Yield Strength: If you tighten a bolt until it starts to stretch (anything over .2% of length) you have reached the fastener's yield point. In some applications bolts are "torqued to yield" so that they act like a spring to hold something together. Aluminum cylinder heads on a cast iron block often use torque to yield cap screws (thread on one half of fastener) These are usually "one time " use only.
So match em up, torque em up, sleep well.
Thanks to Chuck for posting about impact guns, it reminded me to.....