A 'pakset' is the data set that Simutrans needs to run: it contains all the data for the graphics of individual items (vehicles, roads, railways, buildings, etc.) and all of the balancing information (such as prices, speeds, capacities and so forth) necessary to make the game complete.
Pak64 is the main and a base set for Simutrans developed along with Simutrans executable. It always contains the most recent features. It is neatly painted pixel by pixel with motherly care. On the other hand it might look too tiny on high resolution displays. Many artists contributed to this pak set, from the 8 bit age on.