Registers in Vim let you run actions or commands on text stored within them. To access a register, you type "a before a command, where a is the name of a register.
If you want to copy the current line into register k, you can type
Or you can append to a register by using a capital letter
You can then move through the document and paste it elsewhere using
To paste from system clipboard on Linux
To paste from system clipboard on Windows (or from "mouse highlight" clipboard on Linux)
To access all currently defined registers type