DI boxes tend to come in mono or stereo and are either active or passive. Mono means that they have one input and one output most commonly used on acoustic guitars whereas stereo DI boxes have have two ins and outs perfect for keyboards or other sources that have left and right stereo channels. DI rack units with multiple ins and outs are also available to buy for recording studio and live purposes.
Active DI boxes require batteries or 48V Phantom power that is supplied from the connected mixing desk which generates a stronger balanced signal than a passive DI. Passive DI boxes will work without sending power to them but they have a lower input impedance and may require extra gain when setting your levels at the mixer which can cause some unwanted hissing system noise.