Exactly what is...

... a VIRTUAL EVENT?

Moderators, speakers, and participants are in different locations and are only digitally connected. Their communication is purely virtual in contrast to hybrid events.
Synchronous and asynchronous communication elements can be used in their purest forms or combined with one another.

...a HYBRID EVENT?

Hybrid events are live events, which are expanded to accommodate a large audience through the use of digital technologies (e.g. livestream, social media, etc.). This makes it possible for those not present to participate as well.
Here, too, synchronous and asynchronous communication elements can be used in their purest forms or combined with one another.

...a SYNCHRONOUS and ASYNCHRONOUS communication element?

There are a number of different communication elements for both virtual and hybrid event formats. These range from synchronous elements, which refers to all means of interaction where participants can communicate with one another at the same time and live (e.g.: video conferences and phone calls) to completely asynchronous communication tools, where participants communicate with one another with a time lapse (e.g.: emails, SMS messages). There are a number of mixed forms in between, which can be used differently (e.g.: chats, whiteboards).