In the Golden Age of tango (1935-55), there were hundreds of tango orchestras.
Only a handful of orchestra's music have survived the test of time and we still dance to today.

The Orchestra leaders are the namesake of the orchestras.
Orchestra leader is like a movie director, who 

1) holds the creative vision of the orchestra
2) chooses which song to play / record
3) arranges (or directs the arranger to arrange) the score
4) chooses the musician
5) guides the musician in the fulfilment of that vision.
sometimes but not always
6) plays an instrument
7) composes

Each of these orchestras had a distinct style. 
Listen to 30 Essential Tango list to learn popular songs by each orchestras.
We recommend Michael Lavocah's book or David Thomas's book to learn about them. 

Music evolved over times and even the same orchestra sounds different depending on the periods. It's essential as a dancer to learn the different sound of each period/singers/instrumentals.