What is the difference between a Boeing 757 and a Boeing 752?

They are both the same aircraft -

The 757 has 2 main variants: the 757-200 (752) and the 757-300 (753). The 757-200 is the more common variant, with around 1000 built. The 757-300 is longer and is used primarily for charter flights. Only around 50 were built.

The ICAO (Internation Civil Aviation Organisation) uses the codes '752' and '753' to shorten the name to use on radar etc.