We have very little detail about the battle. Most of what we think we know is from the Mozarabic Chronicle of 754, a Latin history by a Christian chronicler which mentions the Europeans defeating Saracens at Tours. (The word it uses for the victors, europenses—"people of the place Europa"—is the first use of the word that became modern "Europe" and "Europeans." Europe is called that because of a Greek myth.)
Modern historians estimate that the Umayyad force outnumbered the Frankish force under Charles Martel by about 80,000 to 30,000, but some maintain that bringing and supporting 80,000 troops into Gaul was logistically impossible. Armies like this had to live off the land, and the land could not have easily supported feeding the tens of thousands involved. On the other hand, scarce rations and the subsequent low energy levels could explain the defeat of a large army forced to fight in unfamiliar territory.
One "casualty" at the time was Odo the Great's army. Although successful 11 years earlier at Toulouse, for the Umayyads to get to Tours they had to go past Toulouse again, which they managed to do. Odo's surprise attack in 721 did not translate to victory in 732: his army was defeated at Bordeaux and Garonne, largely because they were foot soldiers fighting Muslim cavalry. The devastation in southern Gaul this time spurred Martel to gather as large a force as he could.
In fact, Martel had been preparing for this since Toulouse, even though he had chosen not to help Odo. He had taken out a large loan from the Pope Gregory II in order to hire and train and equip a large army. How he used this army, and what we know about how he directed the fighting is a topic for tomorrow.