This circuit was once considered one of the best treks in the world. though road construction is threatening its reputation and its future as a classic trek. Yet no one disputes that the scenery is outstanding: 17 to 21 days long, this trek takes you through distinct scenery of rivers, flora, fauna and above all - mountains. The trek goes counter-clockwise and reaches its summit in Thorung La (pass) at the height of 5416m, or 17,769 feet. The route goes past the following mountains: Manaslu (an 8,000-plus meter peak), Langtang Himal, Annapurna II and IV, Annapurna III and Gangapurna, and, of course, Annapurna I and Dhaulagiri -- passing through the world's deepest gorge in between those two 8,000-plus meter peaks. Poon Hill, at the end of the trek, affords views of those two mountains, as well as South Annapurna and Macchupucchre, the "Fishtail Mountain." The trek also goes through Buddhist villages and Hindu holy sites, most notably the village of Muktinath, a holy site for both Buddhists and Hindus.
The eastern portion of the trek follows the Marsyangdi River upstream, to its source near the village of Manang. To get there several days of up-hill hiking are required. Then the route goes over the pass, a grueling day of hiking, and back down the other side, where it meets up with the Kali Gandaki River. The trek follows this river downstream. At the end of the trek, several options are available: following the river further to the road proper and catching a bus to Pokhara where one hikes up to Ghorepani and Poon Hill, or adding on a trek to the Annapurna Base Camp, known as the Annapurna Sanctuary Trek.
This is a "teahouse trek," meaning there are villages with lodges and restaurants to eat and stay in along the entire route. You are expected to eat in the same lodge where you are spending the night. Prices of rooms are seemingly inexpensive because of this -- lodge owners tend to make more money on the food and drinks they are selling you than on the room where you are sleeping.
While travel agents will say it is necessary to hire a guide and a porter, this tea-house trek is so easy it is not required at all. The path is easy to follow and every three hours (max) you will arrive upon a small village with lodges and restaurants. There are many trekkers around so you always end up meeting people and making friends and trekking partners along the path.
The trek typically starts in the village of BhuleBhule. Buses will get you from Kathmandu to Besishar in a drive that is quite spectacular in itself for the Middle Hills scenery you pass through and the narrow, windy road. From Besisahar to BhuleBhule you will have to take a jeep or an even older local bus. Tourist and local buses are available. But don't let the "tourist" label fool you. Chances are good you'd be the only Westerner on the tourist bus, since they are also popular with the locals who can afford them. They tend to be more comfortable (though it's still a jolting ride), and faster. Local buses take much longer, are much less comfortable, carry very few tourists, but are cheaper, if you're pinching pennies. The ride on a tourist bus takes five or six hours.
Depending on where you end your trek, buses and taxis are available to take you to Pokhara, a great place on a lake to spend a few days unwinding and relaxing. Kathmandu is about a six hour bus ride from Pokhara, easily arranged once you're ready to leave the lakeside town.
You can try and walk the lower stages at a faster pace maybe combining two of them into one so that more time can be spent on acclimatization on the higher stages. For example a couple of extra days can be spent at Manang and utilized by climbing to one of the many peaks around it and coming back down so as to increase the production of red blood cells. You can also see the origin of the Marsyandi river in Manang. The best part about this trek is the varied scenery it has to offer. You start from tropical forest in Besisahar, see terminal moraine near Manang and then move past the snow line across the Thorung La and then to the barren landscape of lower Mustang and Muktinath.
Excerpt from Annapurna Circuit - WikiTravel