A TV tuner allows you to tune in, watch and / or record TV channels from cable OR from an OTA station via an antenna. The better ones have two or more tuners operating independently so you can record one channel while watching another. Many also include input ports so you can watch / record video and audio signals from an external player like a DVD or VCR unit. Some also have audio / video outputs like S-Video or NTSC Composite video, but these are more likely to be a video card feature if you need them.
If you go for multi-tuner capability, read the fine print. Some have two tuners that each are fully capable and share one cable TV input connection. Some have two tuners that each have different capabilities, so that only one can handle cable TV, and the other can only handle signals from an antenna feeding in OTA signals. Be aware that the signal systems for current digital TV from OTA stations and from cable systems are different, so different tuner capabilities are needed for them, but the tuner makers have designed for this.
The tuners able to handle scrambled premium channel and Pay TV signals on a cable system are just beginning to arrive. For a while the transition systems could actually decode some of the scrambled channels on cable for free if you had a tuner card in your computer, but that is about to end with new coding and decoding standards.
For simply watching a DVD or a copy of it on a hard drive, you do NOT need a tuner card. Most of the work for this task is done by your video card and CPU. In fact, even HD playback from a Blu-Ray disk does not need super-powerful video card capability. However, capturing video from cable with a tuner, especially if you are doing it in the background while working on something else, does require a reasonably powerful CPU, and almost certainly a multi-core CPU with appropriate modern OS and application software. That's why False_Dmitry_II recommended a minimum Athlon II X2 CPU. I would bet he / she pointed at AMD systems because they are powerful enough for the job but typically cheaper than Intel-based systems, even if you go higher in the AMD line for 3- or 4-core CPU's.
Regarding copying your DVD collection to hard drives, do some math. A typical DVD movie is around 4 to 5 GB without any of the "bonus features". Your 600-disk collection would require minimum 3 TB to store that IN ADDITION TO the HDD space you use for your operating system, applications and user files. Although HDD's are easily available now in 2 TB sizes, the cost "sweet spot" is still 1.5 TB. So just to start what you plan you will need about three drives of the 1.5 TB size, and maybe at least one more for spare space for the future.
Here's another possibility to consider. What about buying an external HDD enclosure that holds 2 (or 3?) SATA II HDD units, plus 2 or 3 drives of 1.5 to 2.0 TB capacity. Make sure the enclosure has at least BOTH a USB2 interface to the computer and an eSATA interface, and that your computer also has an eSATA interface. Mount the drives in the enclosure, hook it all up via eSATA (a pretty fast interface), and load your DVD collection onto the drives in the box. Now you can record and play back as planned. BUT you also can simply disconnect it, pick up the box, its cable and power supply brick, and take it anywhere to another computer, carrying your whole DVD collection under your arm. Use either eSATA or USB2 to connect to any other computer (even the widely-used USB2 interface should be fast enough for DVD file playback) and play back there. If you do this I recommend do NOT use any RAID capability on the enclosure (if it is included). You will not need the extra speed claimed for RAID0 (it is minimal, and this mode carries much bigger risk of losing ALL the files on the drive), nor the "automatic backup" that RAID1 provides (because it uses up twice as much disk space to achieve this!).
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