Mutt has summarised the differences nicely, and if you remember those, they'll get you through any interview. If you want to add some background to any of the points, then try this (I've built the response around Mutt's summary points, which are in bold).
ANSWER Hmm, Turboprops Vs Regional Jets? . . . . [bow, scrape forelock] . . . . well there is ultimately a dividing line between regional jets and turboprops, and this is determined by both revenue and costs. At the simplest level, jets have Passenger appeal, jets are faster, smoother
But from the airline's perspective, its revenues are determined by a combination of passenger numbers and average fares. Route fare structure, can it support the higher costs of the jets? The circumstances of certain operations means the revenue generated per trip cannot cover the cost of a regional jet. Regional jets have a higher operating cost per hour. They are larger, burn more fuel, have higher maintenance costs, may in certain airlines have higher flightcrew charges, and have higher weight-related user charges in certain circumstances. Capital cost, some of the regional jets cost around $25m. They also have higher finance or leasing costs.
Turboprops provide a lower cost alternative to regional jets, and are the only alternative for airlines with particular operating and economic circumstances.
Route structure and operational performance capabilities. Besides basic unit and trip costs, there are other physical and airline operating constraints which prevent the operation of regional jets. These are limitations posed by short runways, pilot scope clauses preventing the operation of regional jets over certain sizes or in particular numbers, and factors preventing high enough levels of aircraft utilisation.