EDIT Use
The Percent Sign indicates a quantity in its ratio to 100.
It is only used in combination with numbers, not within a text as abbreviation for the word percent.
 
EDIT History
The idea of the calculation of percentages is very old, it can be found already in late Egypt manuscripts and in India in the 4th century BC. Imperator August (ruler of Rome from 31 BC) had taxes on the sales of goods and slaves that were based on fractions of hundred or of twenty.
Both the term "percent" and the character evolved much later in Italy in the 16th century. By that time the trade involving huge amounts of money was common and the 100 had become a common base for mathematical operations. "Per cento" is Italian for "of hundred".

From this term, a number of abbreviations evolved such as per 100, p 100 oder p cento. In an anonymus manuscript from 1425, an uneducated scribe used another abbreviation instead, he wrote "pc" and adorned the c with a little loop. This loop was actaually just used in italian numeration for primo, secundo etc. (first, second, ...) where a loop was put above each number.
From there, the sign evolved to a combination of loop and fraction bar.

It was a gradual process, there are italian manuscripts from 1490 and even 1684 that use the pc with the loop, but also a german document from 1669 with a fraction bar. Since mathematical knowledge usually came from Italy to the rest of Europe this indicates the use of the sign with fraction bar in Italy at an earlier time, too.

The tilted fraction bar can in Germany be found on an invoice from 1836 by A.F.Höschner an one from 1838 by Dietrich and became soon very common. The horizontal bar was pretty much out of use after 1850.
 
EDIT Glyph Shape
The correct glyph uses two small circles, the left one on the top and the right one on the bottom of each side of a vertical line that's tilted to the right.
It should not be confused with the Arabic Percent Sign (U+066A) or the Commercial Minus Sign (U+2052) that both use a slash with two dots instead of two circles.
EDIT Meaning in various contexts
Liquor: amount of alcohol
Traffic signs: slope of the street ahead
Clothing: share of cotton, Polyester etc. a garment contains
Meteorology: probabilities of rain, sun, etc.
Dairy products: how much fat is in the product
Shopping: cheaper, reduced merchandise
ADD NEWSIMILAR CHARACTERS
01 CODE POINT VALUE: : : : : 0025
02 NAME (UNICODE NAME) : : : PERCENT SIGN
03 GENERAL CATEGORY: : : : : Punctuation, Other
04 COMBINING CLASS : : : : : Spacing, split, enclosing, reordrant, and Tibetan subjoined
05 BIDIRECTIONAL CATEGORY: : European Number Terminator
06 DECOMPOSITION MAPPING : : -
07 DECIMAL DIGIT VALUE : : : -
08 DIGIT VALUE : : : : : : : -
09 NUMERIC VALUE : : : : : : -
10 MIRRORED: : : : : : : : : No
11 UNICODE 1.0 NAME: : : : : PERCENT SIGN
12 ISO 10646 COMMENT FIELD : -
 
13  UPPERCASE MAPPING : : : : -
14 LOWERCASE MAPPING : : : : -
15 TITLECASE MAPPING : : : : -
16 DECIMAL VALUE : : : : : : 37
17 UTF-8 HEX VALUE : : : : : 0x25
18 UTF-16 HEX VALUE: : : : : 0x0025
19 UTF-32 HEX VALUE: : : : : 0x00000025
20 XHTML : : : : : : : : : : &#37
21 BLOCK : : : : : : : : : : Basic Latin
22 PLANE : : : : : : : : : : Basic Multilingual Plane (BMP)
23 STROKE NUMBER : : : : : : -
24 RADICAL : : : : : : : : : -
 < CHARACTER >  BLOCK PROPERTIES
U+0025 PERCENT SIGN
DEUTSCH : ENGLISH