Promoters will often offer a number of different ticket types, often with different prices.
A ticket type labeled "Concession" may vary from event to event. It may refer to a student or senior and offer a discounted ticket price. Please note that you may be asked to supply your student ID or senior ID at the door/gate of the event when presenting your a Concession ticket. Most promoters will note what qualifies as a Concession on the ticket purchase page.
If you see an Earlybird ticket available for the event you want to purchase tickets for, then you are looking at a discounted ticket type that is either available until a certain date or until a certain amount of tickets have been sold.
A Group ticket is simply a discounted ticket that is available if a minimum number of tickets are purchased. The minimum number will be shown in the ticket description or you'll see the minimum number when selecting the quantity of tickets. You don't have to arrive as a group; everyone gets an individual ticket, but at a lower price.