It's a search engine that's trying to change the way you search the internet for information. When you Google a word like, say, humor, the only mainstream humorous website that shows in the search results is College Humor (see screenshot below). The rest are all blogs leading to mainstream funny websites.
What you get when you Google 'humor'
However, when you do a 'slashtag' search on Blekko.com for 'humor' (you search /humor), your main search results are mainstream humorous websites like College Humor, The Onion, Lolcatz, and Cracked (see screenshot below).
What you get when you Blekko slashtag 'humor'
Blekko bases it's 'spamless search' on slashtags. They've already made slashtags for generic search terms like 'humor' and 'news' but users need to make slashtags (like /scuba or /biomedical engineering) and add websites/URLs to these slashtags that they would primarily want results from. They can also delete websites from their search results and those will never show up again. Each user's slashtag is unique to them and is completely customized to their preferences.
Blekko adds in the social networking function as well as users can have 'friends' on Blekko and search using each others slashtags. They've also added 'editors' of slashtags, which works on Wikipedia's principle of user generated content being used on a mass/general level. Certain customized slashtags will be able to be accessed by everyone on Blekko.
Blekko went live today, the 1st of November. It's a 20 person startup, chief executed by Rich Skrenta and (obviously) aims to take down Google and Bing. They've raised $24 million to start the website which they got from U.S. Venture Partners and CMEA Capital, as well as leading angel investors including Ron Conway, Mike Maples, Jeff Clavier, and Marc Andreessen. They plan on selling Google like ads based on search words and slashtags