Ribeira means in Portuguese "minor river" or a river with little flow of water. It is also a word for the oldest part of Oporto, a place where the city was born and developed due to trade in the river mouth.
The district was the scene of fights and sieges. When Napoleon invaded
Portugal in the beginning of the XIX century, the stampede of the
fleeing population over an old bridge, located where nowadays D. Luis
bridge is, made it to collapse, causing many deaths by drowning.
Two decades later, civil wars between absolutists and liberals caused a lot of destruction here. Oporto was defending the liberal cause, while Lisbon was absolutist. The center of Porto was under siege in 1832-1833 and many of Ribeira's sites were damaged by war.
Nowadays, Ribeira is a tourist area with many bars, touristy restaurants and terraces, particularly in its main square, Praça da Ribeira (picture)
It is by the river Douro, close to the lower deck of D. Luis bridge and to the center of Gaia.
Thanks to Soulfisher for the text.