or White Hat versus Black Hat.

Onsite optimization, keyword analysis, optimized original content, site speed, all conjure up to making a website great on Google. The thing that made Google great tough was the revolutionary idea they had at the time: links to one site count as votes to the relevance of that site to a certain market. Sure, it's a simplistic way of looking at Google's algorithm, and surely part of that philosophy has rubbed off to all modern search engines.
However, like any algorithm, there are always weak spots. Black Hat operatives have always tried new ways of getting fast results with not much effort. Splogging was born out of this.
Blogs became increasingly popular over the past years (you are reading one now). One reason was that people like to talk about stuff, and read about stuff, most importantly that want to know people's opinion about stuff. The next reason would be that internet based companies see user interaction as a gold mine - new and improved advertising gold mine, mind you. So one and other joined to make internet crazy for interaction. Most generic search engines adapted fast, by indexing and increasing the relevance of such writings. So a link coming from a blog is now a LOT more important than a link coming from a web directory for instance.
White Hatters learned the lesson fast. They will talk with blog owners, get them to review their websites, state their opinions, write their own blogs, generate original content that's  relevant to readers, all in all improving the web while promoting their websites. While Google generally has a dim view of SEO, this way of doing business is considered safe and has the potential of getting out important information. In time, most White Hat SEO experts will raise their own network of interlinked blogs and websites, partnerships that gather to promote all involved. This is called a Link Wheel. Jamie's blog links to Alex's website, that makes a lot of observations about the products sold by X company that.... you get the picture. This takes a lot of time and hard work, but achieves a lot of great results, getting good rankings in time for all involved, that stay there.
Black Hatters  are not willing to make that effort. Most of these guys are those guys you see on the web bragging that "if you don't see results in x amount of time, we'll get you your money back", "Rank #1 in x weeks" or "First page results for just 50$". You've all seem these guys, they're everywhere. Some do Black Hat because they're working on a tight time schedule, some can't be bothered to do better, most don't even know that their client will suffer for what they do. Hell, their client won't even be their client in 2 weeks time, cause the client's looking for that quick fix too.
One solution Black Hatters have in their pockets is splogging. Now what on earth is that? Blogging... with a twist. Google will index blogs very fast, unlike generic websites, but also remove them from index equally fast, also unlike generic websites. But the content is still in the internet archives.... And it was good content, that Google deemed worthy of placing front and center... Why let it go to waste? So a Black Hatter will go digging for those archives, create a few websites on free blogging platforms, copy that content on the websites, get a few anchor links to who you want to promote, and presto! You've got yourself an avalanche of links pouring towards that 2 cents site (cause usually, Black Hatters don't bother with all that SEO nonsense either), and that's how you get all those irrelevant results in your SERP's. The good thing of this methd is that it's fast. The bad thing is ... it will all go downhill equally fast. Fast in Google time is a few weeks/months. Tops.  Google, Bing, etc, all view the satisfaction of the user as the #1 priority. So if that irrelevant content is allowed to survive in the SERP's, the user will migrate to another Search Engine that doesn't. And they won't stand for it.
Major Search Engines review their algorithm's regularly, and also browse regularly the archives. It's easy to spot fakes, and all fake sites and their beneficiaries will get heavy penalties, reaching to total removal from index of said site.
Don't try this at home.