Mauritania is a land of ocean and desert. It is of course no wonder that the main attractions for most tourists are the desert in Adrar and Tagant areas (around Atar), and the ocean in Banc d'Arguin (a natural reserve with dunes ending in the sea, full of millions of birds and protected by UNESCO). The Adrar is exactly how you've always imagined the Sahara: endless ergs (dunes) and regs (rocky desert) with tabular small mountains. Most tourists stay along the west coast of the country, although there are a few beautiful sights far into the interior (rock formations in Aioun, for example). If you decide to travel off the beaten path, leave plenty of time to get around.
Mauritania is an Islamic Republic, and although it is a religiously founded state, most Mauritanians are not extremists, even if the majority of the people in the North are very conservative and quite reserved. The Southern part of the country is filled with friendly people, and they are very welcoming, if a little unused to tourists.
Traveling to Mauritania is becoming easier, with charter flights from France to Atar through the winter. Some charter flights have been canceled this past month due to security concerns. Guides and tourist agencies are quite easy to find. However, Mauritania is not connected to the international banking system. Your Visa card will not work in the local ATM. There are now international ATM's at BNP and Societe Generale in Nouakchott and Nouadhibou. Otherwise, credit cards are accepted almost nowhere. It is easy to change euros, dollars and CFA in Nouakchott, however.