Target grades: what grades you could achieve if you truly apply yourself and work as hard as possible. Predicted grades: what grades you are likely to achieve if you maintain your current effort.
Your target may be an A but if you don't work hard then you'll be predicted a C or a D.
At my 6th form our targets are actually target minimums from ALIS. It basically takes your gender and GCSE results and tells you what the minimum you should get is.
Predictions are what teachers think you will end up with based on effort, AS grades etc