Some hard-drive recorders let you time-shift high-def programs.

Digital video recorders (DVRs) don’t have a slot for removable discs or tapes. They record only on a hard drive much like the one in a computer, and can’t play recorded media. Some combination units pair a DVR with a DVD player/recorder so you can play (and copy to) removable media.

Many DVRs have space for 100 hours or more of standard-definition programming at high quality. Some let you record at lower quality, which raises capacity to 300 hours or more. HD-capable recorders usually hold about 30 hours of HD content.

You can get a stand-alone DVR or one that’s integrated into a digital-cable box or satellite-TV receiver. Depending on the type you choose, you might pay for the service as well as the equipment, either a one-time charge or a monthly fee on top of your cable or satellite-TV bill.

Because they can record and play at the same time, DVRs allow you to pause (then rewind or fast-forward) the current show you’re watching, then pick up where you left off. If you pause a show at the beginning, you can skip the commercials and catch up to the “live” broadcast by the end of the show. Dual-tuner models can record one show as you watch another, or record two programs as you watch a third recorded program.

Many digital video recorders offer recording capability with the convenience of a TV program guide customized for the broadcast channels in your area or the cable or satellite service you have.

If you get your DVR functionality in a digital-cable box leased from your cable company, you’re typically limited to the cable operator’s choice of hardware. For hard-drive recording in a satellite receiver, you might be able to buy or lease the hardware from the Dish Network or DirecTV, or buy it from a retailer.

For stand-alone DVRs, TiVo is the main service provider. Hardware prices depend on whether it’s an HD or SD unit and how many hours of programming you can store. Service charges vary.

You can buy TiVo equipment directly from TiVo or from electronics retailers. TiVo functionality was incorporated into DVRs from Sony, Toshiba, Pioneer, and Humax. Basic TiVo service is included with some of those products at no additional charge, but TiVo’s regular subscription rates apply for full functionality. Price: about $50 to more than $500 for a player that also incorporates a DVD player and VCR. TiVo HD runs about $300 and can record up to 20 hours of HD content. Some TiVo units are free when you sign up for service at $13 per month; longer terms might be available.

Stand-alone DVRs resemble VCRs in size and shape but don’t have a slot for a tape or disc. (The internal hard drive is not removable.) Combination units that add a DVD player or recorder will have the requisite media slots. Depending on the model, it can be connected to your television using HDMI, composite, S-video, component, or possibly RF antenna outputs to match the input of your set.

A digital video recorder’s hard-drive capacity varies in actual usage. Like digital cameras, many DVRs record at different compression settings, and thus at different quality levels. For the best image quality, you have to record programming at the DVR’s lowest level of compression.

To get the maximum capacity advertised, you have to use the highest level of compression, which gives the lowest quality. For example, a model that advertises a 30-hour maximum capacity will fit only about 9 hours at its best quality setting.

The program guide is an interactive list of TV programs that can be recorded by the DVR for the next 3 to 14 days, depending on the guide. You can select a program to watch or record or search by title, artist, or show type for automatic recording.

Ultimately, a DVR’s picture quality, like a VCR’s, depends on the quality of the signal coming in.

Decide whether you need a tuner. If your TV does not have a built-in digital tuner (ATSC) and it is connected only to an antenna, buy a recorder that contains a digital tuner. That will enable you to continue receiving broadcast TV when analog broadcasts end in February 2009.

Do you want the most programming features? The services from TiVo might have more features and functionality than some of the offerings of cable and satellite companies. You might have to buy another box and pay another monthly fee.

Would you prefer to have fewer boxes and service providers to contend with? Inquire whether a cable box equipped with DVR functionality is available. If you use satellite service, consider getting a satellite receiver that includes a DVR.

Keep in mind that you might have to pay a separate fee for the DVR service. And some satellite and cable DVRs work only with the service provider’s programming and won’t record from other sources, such as an antenna.

Do you want to edit recordings or store camcorder video? Then a DVR is not for you. You need a DVD recorder.

Do you want to record HD content? Make sure you specify it before leasing or buying any equipment. Not all DVRs are HD-capable.