I have a customer who has an Aztech wifi modem, Dlink wifi router.
He wants to use the Dlink to extend the wifi signal from the Aztech without using LAN cable in between those two routers. Can those two routers connect with each other using wifi?

Home wireless access points usually do not talk to one another. You need a wireless repeater (a.k.a. range extender) because it is capable of being both an wireless access point and a wireless adapter or "client" to another access point.
Here is a good article about extending your WiFi network:

http://compnetworking.about.com/od/wirelessfaqs/f/extendwifirange.htm

If both units support it, you could use WDS.

WDS sounds good but need to upgrade firmware and might void warranty from supplier. Even so WDS night not work with these two different brand. Some people just can't ask before they buy something.

Thank anyway. Would not have learn about WDS otherwise.