A Passive Splitter works by means of a transformer so that the signal is distributed to each user with the same level (line level) with which it has arrived from the console. It is a device that does not need to be connected to the electrical network to work, so it is very useful for outdoors, for example. Being a passive device there is a small insertion loss.
An Active Splitter has a transformer and amplification or only amplification. In other words, the main characteristic is that the amplification stage will give you the option of modifying the gain by amplifying or attenuating the signal coming from the console. In addition to this feature, it can be equipped with visual monitoring, for example.
In both options, the audio input is balanced by a transformer that gives them a CMRR of more than 100 dB.