In the context to which you are probably referring, both "combinations" and "passwords" perform the same function, i.e. to provide security protection.
In this regard, "combination" is normally associated with locks (for example, bicycle locks or the locks on safe deposit boxes). Also, combinations tend to be numeric, using digits rather than letters. An example here would be the combination 5-4-4-2.
These days, passwords are usually associated with computers and logging into accounts. While some people may use numeric passwords, it is probably the case that most passwords are written either in letters (hence the term "password") or a combination of letters and digits. For example -- johnsmith3567