Hello again.
Recently my old man asked if I could point him in the direction of a source that could explain, perhaps, the difference between 'street art,' 'graffiti,' and 'tagging.' While I follow a few street art blogs and keep up with a few photographers who like taking snappers of colorful walls I'm not certain I know of such a source. And, while said source undoubtedly exists I thought I'd use this opportunity to provide my brief opinion on the subject.
Street art, as I see it, is art in a public place that adds color and/or value to the space it's in. While most street art is done illegally, I wouldn't hesitate to call commissioned work street art so long as it isn't a blatant advert for the side of the business it's probably on. Street art is meant to be looked at, speculated, discussed and enjoyed. Perhaps not enjoyed. Illegal street art often carries with it connotations, political or otherwise. (note: I'm fighting off the urge to hyperlink every word in this post with different examples but that'd take forever and this was supposed to be brief and goddamn I'm the king of the run-on sentence so take a look at the end of the post for a few links to some of my favorite street artists and photographers) It can be meant to incite fear, or anger. That it has meaning is what makes it street art. The most prolific street artists have taken their craft worldwide, some managing to make a living doing what they love. Shepard Fairey/OBEY GIANT and Banksy are probably the two most notable street artists. Banksy, to this day, remains anonymous to the public while Shepard Fairey has built an empire and branded his message on clothing and marketed it as design work.

Shepard Fairey
While I feel there's a stark difference between street art, graffiti, and tagging I will admit that the term 'graffiti' can most commonly be used to describe all three. The differences between graffiti and street art are not that many but are immediately noticeable. Graffiti, I feel, is always illegal. It is meant to 'deface' property though as a fan of some graffiti I have a hard time perennially attaching a negative stigma to it. While it
is illegal it can still provide the same color and/or value to a space that street art does. And while there are many different methods to an artist's product I think graffiti is generally done with more haste than street art. Street art also often takes into account and uses the aesthetics of the space it's in where graffiti is generally the artist's decorative mark added onto the space. Therefore street art is
with the space where graffiti is
on a space. Below are two of my favorite local graffiti artists, Girafa and Musk, who are often found near one another though not exclusively.

Girafa

Musk
Finally, tagging is also graffiti, which is to say that it's illegally defacing a public space. Tagging, however, usually incorporates little or no artistic value other than the difference in the textual shapes of written words. It's mostly characterized only by a written word or series of words, the artist's mark or message. Below is a tag from a local graffiti artist ORFN.

ORFN
There are really so many different kinds of public art and so many grey area's but, hey, I wrote this for a man of sixty f... ahem, 50-something year-old. A man with no prior knowledge. Chew on that, Dad. Lemme know if it makes sense.
Street art resource:
Wooster CollectiveFavorite Artists:
WK InteractOBEYThe London PoliceBlek Le RatSwoonKnow HopePhotographers:
Troy HoldenHabit FormingAnd finally, you can take a look at
a set I've put together of my own photographs on flickr of street art, graffiti, and tagging.
Best,
ad