Whenever you execute an operator, even if that just means to select something, or deselect it, Blender needs to keep track of what you were doing by recording those actions to what is called the Undo Stack. You can think of it as a pile of commands which were run. If you press Ctrl + Z Blender will travel backwards in time along this stack so to speak. So you might be thinking you're not doing anything, but for Blender you do something :)
Think about it: How else would any application know what state you were in before, what the document or model had looked like before you performed a specific action? How would the app be able to go back to previous states, unless recording and keeping those states around?
After performing a few actions, you can see the whole stack by pressing Ctrl + Alt + Z. This will show a menu with the latest commands you executed, and allow you to choose a specific state of the model. The one with the eye icon is the state you're looking at in the 3D View currently:
To verify this, you can simply disable the Undo Stack alltogether by going to the User Preferences, Section Editing and set the number of Undos to Zero (section marked in red):
When you now perform a few actions like select and deselect, you'll see that the memory consumption will jump slightly, but it won't just increase like a one way street any longer. There will be occasions where the memory consumption actually will be reduced, depending on what you do. However, this is strongly discouraged, as you really lose the Undo functionality. You can't even undo selections any more.
The settings in the prefs allow you to set certain boundary conditions on when Blender should forget about what you did. One is the number of steps, the other is the memory limit. Both help you to tailor your personal Blender prefs towards your machine specs. The memory limit is a limit for the Undo Stack itself. So in my case, Blender is allowed to allocate 2 GB of RAM for just the Undo functionality. If I perform a lot of memory intense actions in sequence, this limit could be exceeded, and Blender would start to delete the first element in the stack, then the second, until the stack fits the limit again. Sounds complicated, but bottom line is: Blender will never use more than that, so the constant increasing of RAM consumtion stops at that point.