A PBX is only really useful if you can get calls into it. Of course, you can use Asterisk with VoIP calls (SIP, H.323, IAX, etc.), but you can also talk to the real PSTN through various cards.
Supported Hardware is divided into two general groups: DAHDI devices and non-DAHDI devices. The DAHDI compatible hardware supports pseudo-TDM conferencing and all call features through chan_dahdi, whereas non-DAHDI compatible hardware may have different features.