It's a device that by placing on your guitar, can change the tuning. Example, if you put it on the 1st fret, it raises all the strings tuning by 1 half step. If you put it on the 3rd fret, it raises the tuning by 3 half steps.
A Capo acts like the nut of a guitar. It also raises the guitars pitch by a step/half step.
A clip you place on the neck of the guitar at any fret. That fret now acts as an open note. If you listen to Free Falling (Tom Petty) or Hotel California (Eagles), to name a few, those songs use capos.
Peavey HP Signature EXP (SH4 JB/ SH2 Jazz)
Jackson DK2M
Fender MIM Stratocaster HSS
Fender MiM Telecaster

EVH 5150 III w/ EVH 2x12 cab
Peavey Vypyr Tube 60
Fender Blues Jr.

Boss ME-50; NS-2; RV-5; BF-3
MXR M101 Phase 90
below the capo.

If you put them behind, you're not going to make any difference to the pitch of the string.
Populus vult decipi. Decipiatur.

Quote by Mistress_Ibanez
It's can be a contraction and genitive case.

Quote by Mistress_Ibanez
If you cut down on these costs students won't learn so well, effecting the "quality"...