Pyroelectrics are the bridge between ferroelectrics and piezoelectrics – a Venn diagram showing the interrelations is available here.
They possess a spontaneous polarisation which is not necessarily switchable by an electric field. If their polarisation is electrically switchable then they are ferroelectric, a property that may be exploited e.g. for data storage. Pyroelectrics fill an entirely separate niche.
The pyroelectric effect has been known of for a very long time. The Greek philosopher Theophrastus first noted (in approximately 400 B.C.) that Tourmaline would attract straw and bits of wood when heated. This was due to the fact that the changing the temperature produces surface charges that are capable of attracting other charged materials. Naturally, how this occurred has only recently been discovered, but it is interesting to note how early on the effect was documented.