The term 'echinacea' refers to a genera of plants, and a few species in this family including purpurea and angustifolia are desired due to their alkylamide content (seen as the active ingredients). It's often taken as a tincture, but dry capsules are also common.
Overall, echinacea appears to be somewhat effective for fighting off upper respiratory tract infections (mostly studied for the common cold), and when taken consistently as a prophylactic, it has support from research. It has also been studied for its potential ability to accelerate the rate of recovery in sickness, but the current evidence for that highly ambiguous. There are trials suggesting remarkable recovery rates, and there are trials suggesting no benefits whatsoever. Overall, it doesn't look very potent. Limited research suggests that it can increase the levels of a variety of immune cells, but more research is needed to be confident in this.
Echinacea hasn't demonstrated notably, greater adverse events in trials than placebo, however, as with herbal extracts in general, there may be a risk for drug interactions, particularly for those who take multiple medications.