One general difference between Tai Chi and Qigong is that Tai Chi can be quite complex because it is a series of movements which need to be executed in a specific sequence. Qigong is often a number of largely independent movements and although there may be a preferred sequence they could be done in any sequence. Qigong forms are easier to learn and the complexity of Tai Chi is often a contributing factor to the high drop out rates.
Many of the internet based offerings (including taichi18) are more Qigong than Tai Chi.