You can copy text by using the y (yank) command, followed by a movement or a text object. You can also hit y from a visual mode when you have selected some text.
In addition to all of the standard movement, you can use yy to yank the contents of the current line.
Let's yank the following piece of code by typing ye (yank until the end of the word):
This will copy animal_kind into our default register. Now, place the cursor where you want the text to appear (the text is inserted after the cursor):
To paste the code, hit p:
The delete and change operators also yanks content so that you can paste it later. Oh, and you can prefix the paste command with a number, in case you ever want to duplicate something multiple times.