The simplest and most common type of DI Box is the passive DI. This is generally used to match line level or instrument level sources such like guitars and keyboards and matches them to the mic inputs.
Often, these also have a ground lift switch (this can sometimes reduce unwanted hums and noise).
The drawback of passive DI Boxes is that they reduce the signal level a little (typically from 3-6dB), and can lose a little tone with low-level signals, and sound thin and distorted when overloaded. Active DI Boxes have active circuitry, which means that they require power. This is usually supplied via phantom power or an internal battery.
Whereas Passive DIs use transformers to change signal impedance, the active DI uses powered. A must have for musicians, but if you don’t have, we do.