A tetra is a premixed shake ~ i.e. it comes already mixed in a carton, with a straw which makes it ideal for take outs (for work or if you're off somewhere and don't have chance to mix your normal shake up). Comes in two flavours ~ Banana Bliss and Chocolate Velvet.
They both have a unique taste in that they don't taste like either the banana or chocolate powder shake mix ~ some find them quite sweet as the mix is more concentrated ~ but they are "Lush" mmmmm.
You should be able to get them from your regular CDC, but some charge a little more for them, like the bars.