People blog for all kinds of reasons. I started blogging to help increase environmental awareness with an environmental blog Enviroman Says, then started a blog Blogger for Dummies to provide help for bloggers using the Google Blogger platform for their blogs. I was alerted to the face that "for Dummies" is a trademark of Wiley Publications and thus changed that blog title to "Dummies' Guide to Blogger" and started a new blog Blogger Tips and Tricks which turned out to be hugely popular with bloggers.
However, I am a pensioner and had to support my two sons through collegue with my pension. I had to find a way to supplement my pension and it is a great thing that Google came up with AdSense which provides an easy way for bloggers like us to earn through contextual ads (ads relevant to the content of the blog and thus not irritating to visitors as they may find information or things they are interested in via the ads. In fact, I see may ads I am interested in on my own blog but cannot click on them as that can lead to my account being suspended), referrals for useful things like FireFox with Google toolbar, Google Pack which have products which can make your computer more useful, etc., and from adding Google search box (especially sitesearch box) to our blogs. I find the sitesearch box extremely useful as it helps my blog visitors to search for topics they are seeking.
However, earnings from participating in Google AdSense is directly proportional to the traffic to your site or sites (number of visitors) and some publishers uses Google AdWord to help increase traffic to their site or sites. They uses what is referred to as arbitrage, that is find keywords that people are likely to search for which can result in getting their ads displayed and placing a low bid (the amount an advertiser have to pay when someone click on the ad) to get the ads displayed. This is called PPC (pay per click) ads. By using various means like putting content taht attract high paying ads, they hope to make more from visitors clicking on their ads than the amount they have to pay for their AdWord (PPC) advertising campaingn. Google Adword the the leader in PPC but now Yahoo and MSN is also trying hard to catch up.
Now if you have to spend on PPC advertising campaigns, you will obviously want to try your best to maximise your returns. It will be good to have some tools to help you. This site PPC Tools try to give you everything you need for PPC advertising. This site has some links to some free tools like "Overture Keywords" which will help you find the keywords that people are using to search for information related to your content, and the "free keyword combination tool". It is easy to use the first one, but I still haven't figured out how the second tool works.
There are also some PPC tools for which you will have to pay like the "Google Money Pro E-Book" and "Keywords Elite".
You can also try to get organic traffic (visits from search engines) for which you don't need to pay, and the site Search Engine Marketing Hints also has a post SEO Tools to help you with that.
There are also other topics like "Blog Marketing Hints" and "Social Networking Hints" which can be helpful.