Classic - The bidding is fully handled by the system. The goal is to always place a bid right above the competing bidders but no more than that. The bidding service automatically places bids in your name up to your maxbid. The minimum bidding intervalls vary between items.
Comfort - Works like the Classic bid but with the difference that the system takes the current bid and reserve price into consideration. A good choice if you know you’ll bid high but don’t want to get caught up in bidding below the reserve price.
Active - The bid will be the one you place and is handled manually. Good to use if you want to place a large bid from the start and take home the whole auction, but requires attention from the bidders part in case other bidder might have maxbids in place.
You can read more about the bidding function here.