In ad-hoc networks wireless clients talk directly to each other without any intermediate device. 

In infrastructure networks the APs are working as network bridges and all communication between wireless clients is going through them (even communication between clients associated to the same AP) This allows for advanced functionality such as [client isolation][1]. 

  [1]: https://security.stackexchange.com/questions/16751/wireless-client-isolation-how-does-it-work-and-can-it-be-bypassed