The simple version of way a browser works is as follows:
- Client program (browser) requests web page data.
- Server sends web page data.
- Client program parses the data so that the user can view and interact with the web page.
Note: I know this isn't close to all the requests that the browser makes.
You can view any web page's source code in your browser. There are some ways that a web developer can 'hide' this information but usually this process includes javascript encryption and description. Not every browser can support javascript so it is in the best interest for the web developer to not use this method. If you want to view all the HTML, CSS, Javascript on this webpage all you have to do is (on firefox) click View > Page Source or push Ctrl + U.
Before (or after) the browser parses the information, another program or add-on can remove (or add) certain pieces of code. In this case, AdblockPlus removes advertisements.
The only way advertisers can prevent these types of programs is to control the way client side browser software parses the incoming HTML, javascript, ect. This is a highly unpopular action so I have no idea what advertisers are going to do if AdblockPlus becomes extremely popular.