What do short names / verbs like <leader>, <C-r> mean in Vim?

I'm using Vim for a while now, starting to like it more and more. But when searching for some new features that Vim has in it's goodie bag, I'm often slapped in the face with things like <Leader> <C-r>, etc... I'm feeling really stupid for not being able to relate these to keys, commands, or what soever they should mean. I've also been searching multiple times for the use of these verbs, but I think I'm not using the correct search queries to get to the right explanation pages.

Could someone give me a push in the right direction or maybe summarize the meaning / workings of these verbs?

Solution 1:

:help key-notation lists the meanings of all those <key>.

Solution 2:

Mhhh .. among the first results in google for "vim leader" this pops up:

https://stackoverflow.com/questions/1764263/what-is-the-leader-in-a-vimrc-file

<C-r> is some mapping to the Ctrlr key-combination, whatever that maps to. You just enter :help c-r and press tab to get the list of help topics.