Lucknow is known as city of Nawabs which is also the capital of Uttar Pradesh, the most populous state of India.
The city of Lucknow which is also known as nacklow and lakhnau, was capital of Awadh controlled by Delhi Sultanate and later by Mughal Empire. After the defeat of the Bengal, Awadh and Mughal Nawabs it fell under the rule of the East India Company and later transferred to the British crown in 1857.
Coming to why it is called the city of Nawabs - Lucknow has been ruled by Nawabs of Awadh for a long time who were not only the rulers but also left the impression of their lifestyle on the city which is unique in the world. They were fond of foods and entertainment and use to spend lakhs of rupees at that time. The unique Tahzeeb (Style of living, talking, eating and greetings) of Lucknow is a gift of Nawabs to the city. The people can be seen every where in the city with unique style of communication even the discussion between Rikshawala and Tangewals can surprise any one with the real and sweet Urdu. And many more similar reasons are there which makes Lucknow as a city of Nawabs :)