as far as I know a surrogate key is a special kind of a primary key. It will be used if there is no primary key available.
e.g. if I have the database table "Employees" and every employee in my company has a unique employee signon (or number,etc.) I could use this as the primary key for table employees. This would be a so called natural key. If no unique identificaton is available (or if the unique identification consists of many attributes) an "artificial" attribute can be added to the table as a primary key, holding a value which e.g.comes from a sequence.
Such an artificial pk attribute is called surrogate key...
Surrogate keys are often enforced with a uniqueness constraint. Any surrogate key is capable of being a primary key, although a table can only have one primary key.
Surrogates may also refer to 'natural' keys like social security numbers. In practice though, there's no such thing as natural keys since anything can change (for example, not everyone in this country has a social security number). In large enough systems, a numeric value is often the primary key and other unique fields aren't treated as keys unless you need to build an index on them.