The main difference between an alto sax and a tenor sax is that the alto sax is played in the key of E flat while the tenor sax is played in the key of B flat. While the sheet music for the alto sax doesn't look any different than the sheet music for the tenor sax, alto sax notes are higher than tenor sax notes.
The size of the two instruments also makes a difference in the notes that they play. Since the alto sax is smaller, it creates higher and brighter notes than the tenor sax. Young musicians who have smaller hands and a smaller lung capacity usually have an easier time of playing the alto sax than they do the tenor sax.
Both the alto sax and the tenor sax are known as transposing instruments. What this means is that neither of them sound the same as the piano and other concert pitch instruments. Both instruments also have the same key positions, fingerings and the number of notes they are able to play. Both require a wooden reed and a mouthpiece to play. The alto sax and the tenor sax are the two most popular instruments in the saxophone family. The other two members are the soprano sax, which is the highest, and the baritone sax, which is the lowest.