One spoken language
In everyday conversation, Urdu and Hindi speakers understand each other. Many linguists treat them as two standard forms of one spoken language, often called Hindustani.
The numbers
About 610 million people speak Hindi and about 246 million speak Urdu, as a first or second language. Together, that's more than 800 million people.
The same everyday words
paani (water), chai (tea), roti (bread), doodh (milk), bhai (brother), behen (sister). You'll hear them in Lahore and in Delhi alike.
Where they split
- The script. Urdu is written in Nastaliq, right to left. Hindi is written in Devanagari, left to right. Reading one doesn't teach you the other.
- Formal words. Everyday speech is shared. Formal and technical words split: Urdu borrows more from Persian and Arabic, Hindi from Sanskrit.
So the conversation half of Urdu travels a long way. The reading half is Urdu's own.
Sources: speaker numbers are Ethnologue's figures, as listed on Wikipedia. On Urdu and Hindi as one spoken language, see Hindustani language.