Not all wireless network hardware is compatible. There are four different speeds at which devices can be designed to operate and two different frequency bands. While many devices support multiple combinations of these they do not usually support all of them and the different devices need to have a combination in common that they can use to communicate.
The speeds at which different devices can communicate are known by their protocol names with 802.11b being the slowest, 802.11g being faster, 802.11a being yet faster again and 802.11n being the fastest currently available. These protocols give nominal maximum connection speeds (which you can halve in looking at the real communication speed since data needs to travel in both directions). You also don't necessarily get the fastest connection speed though depending on the signal strength the two devices may negotiate to use a slower speed in order to ensure the integrity of the data being transferred.
While many 802.11g devices also support 802.11b and 802.11n devices will usually also support 802.11a and some 802.11a devices also support 802.11g I have not come across any devices that support all four protocols and so if you want to be able to connect devices using any of the four protocols to your network you may need more than one wireless access point.
This is further complicated by the use of two different frequency bands with slower/older devices being designed to use frequencies in the 2.4 range while faster/newer devices generally use frequencies in the 5.0 range. That devices generally use one or the other of these ranges is perhaps the main reason why devices that support the fastest speeds often do not support those that only support the slowest protocols.
In selecting your wireless devices for your network you need to consider these factors and make sure that the devices on each end of your wireless communication share a common protocol so that they are able to communicate.