If you want to know why there are more process than your tabs opened, read the following simple description.
Chrome's main process handles the UI, tab management and so on. There are usually only one process that do the work.
The renderer handles rendering the page. Roughly, each tab consume one, but some special case make tabs into a single one - same domain uses same renderer, and no more than 20 renderer - newly opened tabs will use existing renderer if exceeded. Renderer Process runs in a sandbox that can increase security by separating in sandboxes. When one renderer crashes, other tabs or the main process will not be affected, and users will be able to reload the page by refreshing.
As plugins should have higher permission than a webpage, and in order to keep Renderer Processes in the sandbox, plugins are separated. Also when one crashes, other plugins or tabs or the main process will not be affected.
Similar to the others, separating extensions into processes is to keep them in sandbox... and a crashing extension will not let the whole browser down.
There is a built-in task manager for Chrome. Press Shift+Esc or "right click on title bar -> Task Manager" to launch it. You can see clearly - and may end processes if one's not responding. Ending processes this way will keep those addresses on your browser, with a "sad tab" replacing it, as mentioned in "Renderer Processes".