Venkatachalam is a ridge (Achalam) near Tirupati on which Vishnu resides as Venkateshvara, the Lord of Venkata. This ridge is Shesha, the world serpent on which Vishnu rests. That is why Venkatachalam is also called Sheshachalam.
Venkatachalam is Vaikuntha (Vishnu’s abode in heaven) on earth.
Tirupati, Venkatachalam, Sheshachalam are the same place.
Venkateshvara, Balaji, Srinivasa are names of Vishnu worshipped there.
Read about the place here
Read about Vishnu here