The SPA-3000 (by Sipura) is an ATA.
Basically an ATA has an ethernet socket on one side (so you connect it to your broadband router) and a telephone socket on the other.
You can programme it via a browser.
Your traditional telephone(s) and (if supported by the VOIP provider) fax machine(s) work directly with the ATA. So, you could theoretically put this ATA next to your broadband router and feed all the phones in your house (or if you've got DECT wireless phones you can use it that way).
Advantages are : no need for a PC to be left powered on, some ATAs have 'fall-back' to ordinary phone lines, some ATAs allow for multiple phone lines via the same interface, some ATAs are very programmable so you can setup all sorts of usage rules. Some providers will sell you an ATA and preconfigure it for you if you're not into doing that yourself.
Watch out for cheap ATAs : they might be locked to a particular provider.
I'm going for an ATA solution so that I don't have to leave a PC on 24-7 and also I like control
Skype is just another provider though they appear to be very propietary. With other VOIP providers : yes, you can simply pick up the phone and use it as normal. If you've got an ATA you don't even need the PC on.