The Marangu route was for many years the standard route up Kilimanjaro. It's one of the shortest routes, and the descent route is the same as the climbing route (which simplifies trip logistics). It's also the only route up Kilimanjaro where you can stay in huts every night, although many people think the huts are in bad enough condition that you're better off in a tent! These factors mean that the Marangu route is usually the cheapest way up Kili.
The Marangu route has acquired the somewhat disparaging nickname "Coca-Cola Route" due to the bottles of Coke and beer for sale at the huts.
In recent years, the Marangu route has received a (deservedly) bad reputation. The Machame route has been promoted as a more scenic and less crowded alternative, to the extent that the Machame route has now become one of the busiest routes.
We don't recommend the Marangu route, but it's the option to choose if you're looking for rock bottom prices and you're confident in your acclimatization (or else not too concerned about actually reaching the summit!)
The Marangu route is usually offered as a 4 night / 5 day trip, often starting out of the town of Moshi. A typical itinerary for the Marangu route is:
Day 1: Drive from Moshi to the Marangu gate (1980 m), then hike to the Mandara hut (2700 m).
Day 2: Hike from the Mandara hut (2700 m) to the Horombo hut (3700 m).
Day 3: Hike from the Horombo hut (3700 m) to the Kibo hut (4700 m).
Day 4: Summit day. Start hiking from Kibo hut (4700 m) at midnight, arriving at Gillman's Point around sunrise (about 6 am). The true summit of Uhuru Peak (5896 m) is another 2 hours or so beyond Gillman's Point. Descend back to Horombo hut for the night.
Day 5: Descend from the Horombo hut back to the Marangu gate and drive back to Moshi.