The system is the part of the universe being studied, while the surroundings is the remainder of the universe that lies outside the boundaries of the system.  It is also known as the environment or the reservoir.  Depending on the type of system, it may interact with the system by exchanging mass, energy (including heat and work), momentum, electric charge, or other conserved properties.  The environment is ignored in the analysis of the system, except in regards to these interactions.