This is a key on the keyboard (in the bottom right and left corners of the main character pad) that is used for special functions.
The control key is usually used with another key. You press and hold down the control key and then press a second key and you will activate a special function.
The keys and their functions all depend on the way the software was written. In many Windows-based applications there is a set of standard control key functions. Ctrl-S will usually save the current document, Ctrl-N will create a new document, and Ctrl-P will print the document out.
To move around any Windows application, Ctrl-right-arrow will move the cursor one word to the right and Ctrl-left-arrow will move it one word to the left. Ctrl-up-arrow will move you to the beginning of the paragraph and Ctrl-down-arrow will move to the end of the paragraph. Ctrl-Home will move to the start of the document and Ctrl-End will move you to the end of the document.
