This fountain is built on the back of a Renaissance palace, with water tumbling over rocks with various statues into a pool. It was started in 1732 to the design of Niccola Salvi but took several decades to complete the work. The central figure is Neptune in his chariot with a Triumph arch symbolising the taming of water. Above are bas-reliefs depicting the origins of the aqueduct - the Romans had a tradition of building fountains at the end of aqueducts - and close to this was the terminus of the Acqua Vergine and its fountain it was badly damaged and was later restored in the 15th century and a new fountain was built. However, Pope Urban VIII wanted some more dramatic and had a fountain built on the current site which was later destroyed to built the current fountain.
Of course there is the tradition to throw a coin in the fountain to ensure your return to Rome. The money is cleared out regularly and used to help the poor in Rome. The fountain may be a stunning baroque example, but the crowds around it make it difficult to get a clear view of it. Its size makes it nearly impossible to get a photo of the entire fountain, but any photo fails to grasp the grandeur of it. Note also there are lots of people attempting to sell various items and if you stand still for a moment they will flock around you.
The narrow streets surrounding the fountain are full of small shops selling cheap tacky souvenirs, if you are looking for such items there is a good concentration of them here with reasonable prices. But the cafes and gelaterias close by are not exciting so hold off and walk a little further.