There are many cities in South India. Below is a list of the nine most notable. Other cities are listed on their specific regional page.
One of the major differences between the South and the rest of India are the languages spoken. The four major languages, Kannada (in Karnataka), Malayalam (in Kerala), Tamil (in Tamil Nadu) and Telugu (in Andhra Pradesh) are all Dravidian languages entirely unrelated to the Indo-Aryan languages, such as Hindi, spoken in the rest of the country. Even the scripts of all four languages, while originally Indic, have diverged quite radically from Devanagari.
As a rule, throughout the South, English is better understood than Hindi. The Tamils, in particular, have resented Delhi's occasional attempts to impose Hindi on them, and a few may actually find it offensive if you try to talk to them in Hindi. Learning a few words of the local lingo, on the other hand, will go down a treat.
Chennai is the main air gateway into southern India, fielding flights from all over the world. However, regional flights connect most southern Indian cities directly to the Middle East (Dubai) and South-East Asia (Singapore, Kuala Lumpur).
Here is the link to temple route map for the travellers visiting Navagraha temples in Tamilnadu. 
South India's Dravidian architecture is quite different from the rest of India. The most obvious, and often striking, feature is the gopuram perched on every temple entrance, a stepped, steeply rising pyramid carved with layer upon layer upon layer of fantastically detailed and brightly painted statues.
Important historical temples include Mamallapuram (7-9th century) and Hampi (14-16th century), while the busiest active pilgrimage sites today are Tirupati, by some measures the entire world's busiest, and Madurai, which has been operating continually for over 2,500 years.
South Indian food is quite different from that elsewhere in the country, being mostly rice-based. They also make greater use of pulses. The typical meal is sambhar or koottu (a watery curry) with rice, or avial (mixed vegetables) with rice. There are regional variations too — the coastal regions make greater use of coconut and fish. In the coast, it is common to use grated coconut in everything and use coconut oil for cooking, while someone from the interior could be surprised to learn that coconut oil, can in fact, be used for cooking.
A very incomplete list of typical standalone Southern dishes, all of which are commonly eaten for breakfast:
All of these can be eaten with dahi, plain yogurt, and chutney, a condiment that can be made from practically anything. South Indian cuisine is predominantly vegetarian, though Chettinad, Andhra and Kerala cuisines use meat heavily and are a lot more spicier. Coffee (kaapi in Tamil) tends to replace tea in the south.
A South Indian speciality is the banana leaf meal, served on, you guessed it, a banana leaf. This consists of steamed rice served with about two to six vegetable dishes like sambhar, dry curry, rasam (a thin, peppery soup), koottu along with curd and buttermilk. For a non-vegetarian meal, curries or dishes cooked with mutton, chicken or fish are included. Meals are often accompanied by crisp appalams. Refills of curry and rice are often free, with men with buckets walking around to serve you more. After a final round of rice and curds or buttermilk or both, a traditional meal is concluded with a small banana and a few betel leaves and nuts. If served on a metal tray instead of a banana leaf, a set meal like this is known as a thali instead.