Homestar Runner (often abbreviated as HR, HSR or H*R) is an internet cartoon that can be found at homestarrunner.com. The title character Homestar Runner and fellow main characters, the villain Strong Bad, Homestar Runner's friend Pom Pom, and Strong Bad's lackey The Cheat, first appeared in the fake children's book parody "The Homestar Runner Enters the Strongest Man in the World Contest". The book written by Mike Chapman and Craig Zobel in 1996. Now, them and nine other main characters thrive on homestarrunner.com, run by "The Brothers Chaps", Mike and Matt Chapman. The site is perhaps most popular with (and has been gravitating toward) young adults.
The cartoons ("toons") nominally center on Homestar Runner, a somewhat dim but good-hearted athlete. Strong Bad, however, is often more popular among fans, mostly through his near-weekly updates of Strong Bad Email before the several-year hiatus. Strong Bad Emails are short cartoons in which he answers actual emails from viewers, answering questions like "What would you look like as a japanese cartoon?" and "Can you do a magic trick?". Strong Bad works closely with his sidekick The Cheat and uses his brother Strong Mad as the muscle in his operations. Together, the three prey on Strong Bad's depressed brother Strong Sad. Many of the site's features — games, main pages, characters, music, etc. — are based on things from the Strong Bad Emails.
Several other characters fill out the world of Free Country, USA, where most of the characters live: for example, Homestar's hippie girlfriend Marzipan whose answering machine is a frequent target for prank callers like Strong Bad. Other main characters include the verbally-challenged coach/janitor/rapper Coach Z, local concession stand owner Bubs, The King of Town and his Poopsmith. Rounding out the cast is Homsar, an odd-speaking character created on account of (and in mockery of) a misspelling of Homestar in a Strong Bad Email. Many of these characters have alternate versions, like Old-Timey or 20X6.
You should check homestarrunner.com out first before you check out the fanstuff on the wiki, or else the fanstuff might not make much sense. The Homestar Runner Wiki can be found here; you should check that out too.