A central reservation system (CRS) is a platform used by hotels to centrally manage and distribute room inventory, rates, and reservations. The CRS extracts rates from your PMS (property management system) and then writes back reservations into the PMS when booked across a variety of channels like GDS and IBE. IBE stands for internet booking engine and is a somewhat dated term. Modern hoteliers have taken out the word "internet" because frankly the whole world is the internet today and it's a bit redundant. The easiest way to think about a booking engine is it's basically the shopping cart for a hotel website where (direct) bookings and payments happen.