First off if your Acer has a spare PCI slot then the card would be OK.
ADSL and cable routers/modem are certainly different.
ADSL is nicknamed a last mile technology, as ADSL is ran from your local exchange to your home via twisted pair phone line. ADSL and the voice calls use different frequencies and are filtered in the home, using ADSL filters.
Cable BB is a shared medium, like ethernet, that uses fibre cable from the Cable Modem Termination System (think of it as a telephone exchange for cable) to your home. The fibre cable also carries T.V. and voice calls. Most fibre is terminated and a co-axial is brought into the home to the cable modem which splits the signals.
Conclusion - ADSL and cable are two very different technologies, therefore thier modem are to.
Dongles are just a nickname given to a USB network adapter, thats all.
I hope this helps you understand - even if just a little bit better :D