Time needed: 2 – 5 min
Why? To structure and organize a given task, or evaluate students foreknowledge about a new topic.
When? Any time when you introduce a new topic or you give students to work at a new independent task to give them the chance to structure their work and find cross lines between different topics.
How? Its another way of brainstorming, just place the topic in the middle and let students give to you terms which they know about this topic already and place them around the main topic. A tag-cloud is just for collecting different connected terms to one topic, there is no need to place it in a certain order. In higher levels you could start to give those term order already, and develop a mind-map.
Any risks? It could be very time consuming, just give a clear time frame to finish the tag-cloud, it is a task which needs spontaneous and quick action.