These are internal modems, but there is more that one per card. 8 modems/card is common. You will need these if you want to have several people simultaneously dial in/out to your computer. Note that these modems are not digital modems and will thus not be able to use 56k for people who dial-in. So they are not suitable for ISPs. The cards listed claim to work with Linux and the websites should point you to a driver for them.
The use of modems by ISPs is not covered in this HOWTO since it's become somewhat complex due to the introduction of 56k modems which require a digital connection to the phone company. See What do I need to be an ISP?. Cyclades promotes their own products here so please do comparison shopping before buying anything. Also bear in mind that one may turn a PC into a "remote access server" by buying expensive specialized PCI cards containing digital modems, etc.