What are QR codes?
They are 2-dimentional bar codes, that can encode significantly more data than a "normal" barcode. A common application of QR codes is the encoding of a URL. A printed QR code acts therefore as a "link" between a printed format and an internet address.

How can I produce a QR code with my data?
You can generate it with this link: http://do.barcodify.me?msg=<my data>

How can I scan a QR code?
There are tons of barcode scanner apps for smartphones.
Here is one for Android phones, and here is one for iPhones.

Enough of QR codes, how do I generate a Code 39 / Codabar / EAN-8 / xxx  barcode?
Look here for a list of supported formats and how to use them.