When one of your customers makes a purchase, you purchase the product from a third-party company (the drop shipper, usually a manufacturer or wholesaler) for a lower price. This process is as simple as forwarding the order from your customer, a process that can actually be completely automated. (Remember you don't have any risk here of buying inventory because the sale has already been made).
In the long-term, it affects your versatility, functionality, and, of course, your brand. In the short term, it can certainly add/take away a lot of headaches. That said, just like choosing a physical house or office, there is no such thing as an absolute “best” or “top” choice. There’s only the right choice relative to your goals, experience, and circumstances.