Frontlist titles are titles that have gone on sale in the current year. This is a somewhat flexible definition, as I'd still consider titles that went on sale in December 2009 to be frontlist titles; for this reason, "frontlist" often refers to titles that went on sale less than a year before the current date (in today's case, after 2/25/2009). Backlist titles, then, are any titles that are older than frontlist titles, i.e. went on sale a year or more before the current date.
Based on the above definitions, it should come as no surprise to you that the majority of a publishing house's revenue comes from its backlist. This is because 1.) for any well-established house, the backlist invariably consists of more titles than the frontlist, and 2.) the backlist generally contains the "tried and true" perennial sellers upon which the house's financial durability is built. True, the frontlist is where the bestseller action initially occurs (e.g. appearances on bestseller lists such as the New York Times'), but there are only so many bestselling books and "future classics" per year; over time, they accumulate in the backlist, where they continue to slowly but steadily sell. Frontlist titles that don't sell eventually go out of print (though this often takes longer than a year, so most become backlist titles for at least a short period of time).
In terms of sales modelling, frontlist titles (if they are successful) generally peak within the first twelve weeks (often earlier) and decline predictably, assuming there are no major outside influences at work (which there often are, e.g. winning the Pulitzer Prize or being selected for Oprah's Book Club). Once a title becomes a backlist title, it generally reaches a kind of equilibrium and continues to sell steadily for years. There are some exceptions, however, for books that are seasonal/occasional (e.g. books like The Polar Express seeing a sales spike around Christmas) or are considered classics (guys like Salinger and Shakespeare see sales bumps in September, when everyone goes back to school). Again, as you might imagine, the backlist is generally more predictable than the frontlist; aside from the death of a famous author, the backlist isn't perturbed by the same market forces that can make the frontlist so volatile.