Bosphorus is the name of the straights that separate Asia and Europe. It looks like a wide river flowing from the Black Sea to the Marmara Sea. It is fairly recent, dating to about 5600 BC. With new discoveries of neolithic settlements in Istanbul dating back to 6400 BC we now know that people actually walked from Asia to Europe back then.
Bosphorus is very important for trade and political strategy and has been the target of foreign empires throughout history. Gallipoli War was an attempt by the British Empire to cross Canakkale (Dardanelles) straights and then control Bosphorus and Istanbul. After Ottoman Empire lost the war along with its ally Germany in WWI, British ships had invaded the Bosphorus in 1918.
Bosphorus is the most important natural beauty of Istanbul. From Kadikoy in the Marmara Sea in the south to Anadolu Kavagi and Poyrazkoy at the Black Sea north the Bosphorus offers visitors a wonderful view of seaside mansions, woods, hills, palaces and mosques. There are many ways to do a Bosphorus Cruise for different budgets and durations. A guided tour would be beneficial to not only indicate the places of interest but also the history behind them.