Here is a link to a leaflet which is a guide to infant formula.
Some basic info from there:
What types of infant formula are there?
First milks
These milks are often described as for newborns. They are based on the whey of cow?s milk and are more easily digested than the other milks. Unless your doctor or health
visitor suggests otherwise, this is the best type of infant formula for your baby.
If bottle?feeding, first milk is the only food your baby needs for the first six months. After six months continue to give first milk as you start to introduce solid food. When your baby is one year old, ordinary (full?fat) cow?s milk can be
given.
Second milks
These are often described as for ?hungrier babies?. There is no evidence that babies settle better or sleep longer if given
these milks. They are based on the curd of cow?s milk and take your baby longer to digest than first milks. They are not
recommended for young babies.
Follow?on milks
Follow?on milks are described as suitable for babies from six months of age. It is not necessary to move your baby on to these milks.
Follow?on milks should never be used for babies under six months old as they are not nutritionally suitable.
However, the labels can look very similar to first milks so read them carefully.