Drop-Catching is the process of registering expired domain names by means of automated systems within a short period of time (even a fraction of a second) after being canceled by the registry. Simply put, the auto-registration of domains in the moment of their deletion. Drop-Catching is performed by various companies known as "drop-catchers". 
The process of drop-catching is different from registering expired or new domains. The main objective of drop-catching is to secure expired domain names once these are dropped from the domain registry. This process depends on instantaneous actions, so domainers must act as soon as possible in order to secure these domains.
Drop-catching is usually used either to regain a domain or with the purpose of swiping up high-quality and successful domains for resale.
How Does Drop-Catching Work?
Instead of waiting for a dropped or deleted name to become widely available a domainer could place a back-order with a "dropcatcher", who will secure the domain for a fee.
Due to the competitiveness between drop-catching companies and the differences between drop-catching services, not all back-orders are successful. In the case of multiple bidders for the same domain, a private auction will be created. The prices for back-order may vary, depending on the situation, and whether it involves an auction or not. In the case of a single bidder, the price of a back-order is around $60-$70. But in the case of auctions, the price can increase exponentially.
Drop-Catching Software Programs
However,lately, there were also developed various forms of software programs that automatically register the dropped domains in the second these become available; which provides the same assurance and seemingly more reliable results than a back-order service.
Drop-Catching and ICANN
Drop-catching takes advantage of ICANN's Add Drop Grace Period. Having this in mind, drop-catching is considered by many a serious abuse of domain procedure, similar to domain tasting, domain kiting, or typo-squatting.