G is the seventh letter of the English alphabet, the stock symbol for Gilette, and there's even a movie titled G, which I'm told came out in 1974. In English, it commonly represents a voiced velar plosive stop (IPA: /g/), but also represents a voiced palato-alveolar affricate (/ɟ/). Written after the letter N, it represents the voiced velar nasal (/ŋ/), and when appearing before the letter H, can be either silent or pronounced as a voiceless labiodental fricative (/f/). More interesting and less technical than all this, however, is the story behind where this letter came from.
Originally, back when people spoke Phoenician (around 1000 BCE), the /g/ sound was represented with a letter that looked like a large caret (^) or upside-down V which was called gimel ("throwing stick") or gaml ("camel"; so named because of the camel's hump?) in the Semitic languages. When the Greeks adopted the Phoenician writing system, they modified the shape into something like an vertical-flipped L (Γ or Γ) and called it "gamma," still using it to represent the /g/ sound. Before long, the system reached Italy, and they used the gamma for a /k/ sound because they didn't have a /g/ sound. When the Romans came along, (after changing the shape slightly to something between a large < and a (, which is even in Unicode as U10302, in the Old Italic set U10300) they also used the gamma for the /k/ sound. However, they did have a /g/ sound, and using the current shape for both sounds wasn't working. In 312 BCE, Spurius Carvilius Ruga added a stroke to it to create what is now the letter G. The original shape was used to represent the /k/ sound, and the new G shape was used for the /g/ sound. The new letter took the place of the Z, which was deemed unnecessary, and was tossed (only to be reintroduced later).
Those crazy Romans. Inventing letters and whatnot.
For the curious, the serif at the top right of the Latin script lowercase G (you know, the typewriter-style G, with the two loops) is called the "ear." The top loop is the "closed counter," the bottom one is the "closed loop" and the bridge that connects the two is cleverly called the "link." The upwards serif at the top of the capital G and the leftwards serif at the top of the interior line are "bracketings," the downwards serif at the top of the letter is the "barb," and the downwards serif at the base of the interior line is the "spur."
Ask a Linguist: http://linguist.emich.edu/~ask-ling/archive-most-recent/msg00507.html
Encyclopedia Britannica: http://www.britannica.com/ebi/article?eu=296363
deviantArt, 'The Typographic G': http://www.deviantart.com/view/1192676/
Muke's writeup in International Phonetic Alphabet