With reference semantics, assignment is a pointer-copy (ie: a *reference*). Value (or 'copy') semantics mean assignment copies the value, not just the pointer. C++ gives you the choice: use the assignment operator to copy the value (copy/value semantics), or use a ptr-copy to copy a pointer (reference semantics). C++ allows you to override the assignment operator to do anything your heart desires, however the default (and most common) choice is to copy the *value*. Smalltalk and Eiffel and CLOS and most other OOPLs force reference semantics; you must use an alternate syntax to copy the value (clone, shallowCopy, deepCopy, etc), but even then, these languages ensure that any name of an object is actually a *pointer* to that object (Eiffel's 'expanded' classes allow a supplier-side work-around).
There are many pros to reference semantics, including flexibility and dynamic binding (you get dynamic binding in C++ only when you pass by ptr or pass by ref, not when you pass by value).
There are also many pros to value semantics, including speed. 'Speed' seems like an odd benefit to for a feature that requires an object (vs a ptr) to be copied, but the fact of the matter is that one usually accesses an object more than one copies the object, so the cost of the occasional copies is (usually) more than offset by the benefit of having an actual object rather than a ptr to an object.
There are three cases when you have an actual object as opposed to a pointer to an object: local vars, global/static vars, and fully contained subobjects in a class. The most common & most important of these is the last ('containment').
More info about copy-vs-reference semantics is given in the next questions. Please read them all to get a balanced perspective. The first few have intentionally been slanted toward value semantics, so if you only read the first few of the following questions, you'll get a warped perspective.
Assignment has other issues (ex: shallow vs deep copy) which are not covered
