Can some explane bolts for me (m5, m6, m8... what's the difference)?
 

I understand threads per inch, standard bolt sizes, but what the hell does this M# mean?

M=metric and the following number is the thread diameter in mm. Thus M5 has a 5mm diameter thread (not the diameter of spanner you need). An M5 needs an 8 mm spanner.

The "M" means "metric."

Metric threads are designated by diameter and thread spacing. For instance, the most common bolt size used on bicycles is 5 x .8, also known as "M5." This size is used for water bottle mounts, cable anchor bolts, fender/rack eyelets, shifter mounts etc.

The "5" means the bolt diameter is 5 mm. The ".8" means that the threads are .8 mm apart.

5 mm diameter hardware with different thread pitches also exists, and is occasionally seen on older French stuff and the like. The "M" designation is shorthand to indicate the standard coarse thread pitch (.8 mm in the case of "M5")

M6 is 6 x 1 mm threading. This is the second most common threading on bikes, used for brake mounting bolts, threadless stems and other things.

M8 is, I believe 8 x 1.25 mm threading though, again, there are other pitches used in the 8 mm diameter.

As another example, chainring stack bolts are commonly 10 x 1 mm, which is a fine thread, so they would not be called "M10", since "M10" refers to the 10 x 1.5 mm size, not commonly used on bicycles.

Sheldon "Numbers" Brown

I'm glad you mentioned the fastener size is not the same as the spanner size, a fairly common error.

For the Americans, "spanner" = "wrench".

Sheldon is correct that there are different thread pitches for a given bolt diameter in metric sizes just as there are two "standard" series of English bolts, NC and NF threads. For example the most common 1/4" bolt is 1/4-20 (an NC size) or 1/4" diameter by 20 threads per inch but there is also a 1/4-28 in the NF series.

As noted, the "standard" 8 mm bolt is 8 mmx1.25 mm but the crank bolts on square taper bottom brackets are 8 mm x 1.0 mm.