Party votes
This is the number of votes for a given party, and can be a raw vote count or a percentage, as long as all entered values are proportional to each other.
FPTP seats
This is only relevant to additional-member systems where the allocation of seats takes account of party wins within separate constituency seats. The algorithm used is that employed by elections to the Scottish parliament, where the number of constituency seats won by a party is added to the denominator for the party.
In election results lists, the margin shown after the last seat results is how short a party is from gaining the last seat. This is clearly zero for the party that actually gained the seat, but I know a lot of interest is often who has/had the best chance of snatching last place.
Party seats
The number of seats won in the election calculation. Note that this excludes any FPTP (i.e. constituency) seats the party make have.