A "regular bid" is a one time bid by the bidder in the exact amount. For example, if the increment is $25, and the next bid would be $75, you could bid the exact amount or anything higher; $76, $100, etc. By using the "regular bid" you are letting everyone know that this is what you are willing to pay for the item.
An "autobid", which is more convenient, is when the bidder puts the maximum amount he or she is willing to pay. The bidder's amount will only be increased if another user bids against that item. If the increment is $25, and the next bid is $75, you have an auto bid of $100, it will only show $75 is the next bid. No one knows what your maximum amount is. When another user comes to bid, they will get a notice that he or she has been "outbidded" due to the autobid. The user's amount with the autobid will have increased.
At any time, if by chance you do get outbid using a regular bid or an autobid, you will receive an email if you checked the box to be notified in your registration page, and will have the option to place another bid on the item.