A creole language is not necessarily a combination of two languages. It's just a pidgin language that has native speakers.
A pidgin language is usually a limited, easy-to-learn language used for communications between two different language groups. Normally it has much simpler vocabulary and grammar. The hallmark of a pidgin language is that nobody speaks it as their primary language... it serves as a lingua franca (shared language) between two linguistic communities.
Pidgins are not necessarily anyone's primary language. These days people often use basic English as a pidgin even when neither of them is really an English speaker (for example, a Tagalog-speaking sailor might use basic/pidgin English to communicate with an Arabic-speaking merchant when in port, or a Russian tourist in Japan might use basic English to communicate with her hosts).
Sometimes pidgins are used so much that they become primary languages, and kids grow up speaking only the pidgin language. At that point the language is considered a creole language. The classic example of this is Haitian Creole, which started out as a simplified version of French used by African language speakers in Haiti, and soon became the primary language kids learned. Another major example is Tok Pisin in Papua New Guinea.
Yiddish is none of the above, really. It is mostly an in-group dialect that became a full fledged language when it got its first army (Joke!) It was not intended to be used as a lingua franca, although sometimes it served this purpose when Jews from different linguistic communities (e.g. Germany and Poland) used it as a common language.