To make C as junction object, create a master detail relation on C Object pointing to A and B. Hence C object will have two master detal relationships /fields acting as a junction for A and B.
You could consider this scenario,
An organization has multiple open positions (stored in object A), and a candidate (stored in Object B) can apply to more than one positions (which is Job application stored in Object C). Hence c object stores the information on corresponding candidate and the position he/she applied for. (Giving Many to Many relationships)
Lets say, we have three objects. A, B, C
To make C as junction object, create a master detail relation on C Object pointing to A and B. Hence C object will have two master detal relationships /fields acting as a junction for A and B.
You could consider this scenario,
An organization has multiple open positions (stored in object A), and a candidate (stored in Object B) can apply to more than one positions (which is Job application stored in Object C). Hence c object stores the information on corresponding candidate and the position he/she applied for. (Giving Many to Many relationships)
A - Position
B - Candidate
C - Job Application
Hopefully this clears your question.
Regards,
Bharathimohan Rammaurthy
Salesforce For All (http://salesforceforall.blogspot.com/)
https://help.salesforce.com/HTViewHelpDoc?id=relationships_manytomany.htm#JOJunctionObject