Almost all reel-to-reel tape decks have two RCA connectors on the back or sides. Two RCA connectors are line level (auxiliary level) outputs, that can connect to any auxiliary level input (tape, DVD, CD, VCR, etc) on your stereo. Two other RCA connectors are line level inputs, which can connect to the line output of your stereo (tape out, video out, etc). DO NOT connect the line inputs of your reel-to-reel to the speaker outputs of your amplifier or receiver. Serious damage can occur to your reel-to-reel or your stereo as a result.
Some reel-to-reels have a 5 pin (or several 5 and/or 3 pin round connectors instead of the RCA connectors, or in addition to the RCA connectors. These round connectors are called DIN connectors, and were generally found on European model reel-to-reels, although some Japanese manufacturers also had DIN connectors on their equipment (Akai, for one). You can buy DIN to RCA adapter cables, where one 5 pin DIN connector breaks out into two or four RCA connectors, If you have a 5 pin DIN at the back of your stereo amplifier or receiver, you can use a 5-pin DIN male to male cable to connect both the record and playback cabling with one connector.