There's a good reason why Google is a kazillion dollar business: Adwords. Facebook's share prices rose dramatically not becuase of new features, because their advertising revenue went up.
Google is a free search engine funded by the dizzying profits from their Search Advertising (and display adverstising to a lesser extent). Millions of businesses around the world compete in a constant battle to win the online auction just to get your clicks.
Because Adwords is a live auction you cannot "set and forget" your Adwords Campaigns and wait for the money to roll in... you're fine if you want to wait for the money to roll out - that bit's easy.
As well as the bidding changes there are shifts in keywords and visitor patterns – you need to keep on top of what's changing. A negative keyword can start stealing clicks – clicks you're paying for. Clicks that blow your budget and take away from your valid clicks.
Quality Score will have a massive impact on how much you pay per click - Google will reward you for high quality content with a lower Cost per Click (CPC) and you get higher up the page in the Search Engine Results Pager (SERP).
So, in short, it's a minefield. If you want us to review your Online Advertising then please get in touch. One customer changed their Cost Per Acquisition from over $90 per Lead to $45 per Lead in one session.