x + y + z represents a sum of terms.
Plus has attributes Flat, Orderless and OneIdentity.
The default value for arguments of Plus, as used in x_. patterns, is 0.
Plus[ ] is taken to be 0.
Plus[x] is x.
x + 0 evaluates to x, but x + 0.0 is left unchanged.
Unlike other functions, Plus applies built-in rules before user-defined ones. As a result, it is not possible to make definitions such as 2+2=5.
See The Mathematica Book: Section 1.1.1.
See also: Minus, Subtract, AddTo, Increment.