One of the best things about vim is its ability to create custom mappings for almost any key in all modes.

Suppose you want Ctrl+S to save your document while you are in insert mode, because pressing

<Esc>:w<Enter>i

to switch to normal mode, save the file, and return back to insert mode is a bit too much. According to vim philosophy, it actually is too much, and thus you can map Ctrl+S to those keys just as easily.

For this, add the following to your vimrc

inoremap <C-S> <Esc>:w<Enter>i

EXPLANATION:

The command has separate parts but follows the format

<cmd> <lhs> <rhs>

The command maps lhs to rhs in the mode specified by cmd. <cmd> can be further broken down in the following format:

<mode>[nore]map

where the following modes are common (All modes can be viewed here or using :help map-modes)

(none): normal, visual, operator-pending mode
i: insert mode
v: visual mode
n: normal mode

nore is used for no-remapping, i.e. the default functions of rhs and not their aliases will be used. When skipped, The rhs will be further resolved if a remapping is found. Vim, even has a max resolve-depth in case of an infinite loop.

For lhs and rhs the keys are named as follows:

<C-a> : Ctrl + a
<S-a> : Shift + a
<A-a> : Alt + a
<C-S-a> : Ctrl + Shift + a
<Enter>
<BS> : Backspace
etc..

The whole list can be viewed here or using the :help key-notation command inside vim.