Prospector Lite provides two very useful functions. First, it is a powerful search tool, allowing you to create multiple categories and sub-categories of searches, grouped however you see fit. This is great for people with lots of different interests, or for families--you can have Mom's searches, Dad's searches, and so on. The searches can be very highly customized--only in a given category, only auctions, only with 'Buy it Now', and many other options. Searches can be sorted by price, bids, or end date. Prospector Lite limits you to 30 searches per day--enough for most users.
But finding is only half of the equation--there is also getting. For this, Prospector Lite allows you to place bids at a specified time interval before the close of the auction, defaulting to five seconds. (You can also bid directly, of course!) The Prospector server will send your bid to eBay at the instant you specify. This is no guarantee of success, of course--a lot of people are using sniping tools and someone else might get there first--but it helps. This free demo version allows 15 'snipes' a month. There are several commercial versions: Prospector Searcher allows unlimited searches and 30 snipes; Bidder allows 60 searches and unlimited snipes; and Professional has no limits at all.
Prospector's strength is in its searching--it's a tool for those who hunt for a lot of things on eBay and need those searches regularly updated. It's free, so there's no reason not to use it even if you only buy one or two things a month, but it's most useful if you're a constant visitor to the international garage sale that is eBay.