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→Hardware/Physical Connections
Line-In inputs are typically a pair of balanced 1/4" jacks and found on the rear of interfaces. Other formats would include the unbalanced red and white RCA jacks used on consumer devices, which is useful to connect a CD or mp3 player. They are designed to accept a stereo signal, but can be used to input 2 separate mono inputs.
SPDIF input/output is also found on the rear of the interface as a pair of jacks. These may be optical, coaxial, or both. Coaxial appears similar to RCA jacks but using an orange color rather than red or white. Although the connection is identical to RCA, RCA cables are typically a different resistance as [[Choosing Cables#Digital|cables designed to transport SPDIF digital audio. See [[Choosing Cables#Digital]]- standard RCA cables may work, but may intermittently lose their sync...or they may not work at all. Optical cable is often referred to as TOSLINK to avoid confusion with coaxial cables and use a different jack. I don't find optical cable to be not worth the extra expense for home recording, but it's generally not that expensive and often thinner than coaxial cable, creating less clutter.
For outputs, interfaces typically feature pairs of balanced 1/4" analog outputs, one of which is likely to be used for studio monitors. Others can be used to feed external gear, such as external effects or a guitar rig for reamping. There's also SPDIF out which is useful when you need to send your signal to other digital gear - this bypasses the set of D>A and A>D conversion that would happen if transmitting via analog cables.