If you have two devices with wireless network adapters- whether wireless enabled PDA's, laptops or desktop computers- you can technically connect them in an ad hoc wireless network. Essentially, any devices enabled with wireless network adapters within range of each other can all communicate with each other over such an ad hoc network.
If you want to build a more traditional network- where multiple clients connect to a central point which acts as a gateway to the rest of the world- you will also need a wireless router or wireless access point to implement an infrastructure mode network.
A wireless router has a physical network connection which would be connected to a wired network. It could be connected to a hub or switch on a network or for home users would generally be connected to your cable or DSL modem. All of the wireless-enabled clients would then connect to the wireless router. Typically the wireless router will act as the DNS server and Internet gateway for the clients attached to it and it can also be configured to provide IP addresses automatically using DHCP.
A wireless access point (AP) can be used to join wireless devices to a wired network, or to extend the range of a wireless network. They don't provide the DNS, DHCP, firewall or other functions commonly found in wireless routers. They simply take a wired or wireless network input and relay it to the wireless devices within its broadcast range.
To summarize, the primary components of a wireless network include:
