Six months ago, with great fanfare, India launched its first nuclear submarine, the INS Arihant (Destroyer of Enemies). This came after over a decade of planning and construction. India has since revealed that the nuclear power plant for Arihant is not yet operational, and it may take up to a year to get that taken care of. Then will come a year of sea trials, followed by the commissioning of Arihant into service. Even then, Arihant will not be a regular member of the fleet, but a "technology demonstrator" ship. That's why Arihant has only four silos for SLBMs (sea launched ballistic missiles). Arihant will be used to develop and test firing SLBMs while submerged. India's existing SLBM, Sagarika, has been test fired by silos fitted to pontoons, but appears too large to fit into the Arihant silos.
The Arihant is based on the Russian Charlie II sub, which it resembles. Russia retired all its Charlie class subs in the early 1990s. India leased one from 1988-91, and gained a great deal of familiarity with it. The Charlie class had eight launch tubes, outside the pressure hull, for anti-ship cruise missiles. The Arihant has four vertical missile silos. The exact purpose of vertical launch tubes on the Arihant is unclear. The navy revealed very little detail on the new sub (which, until two years ago, the government refused to say anything about.) Access by photographers was restricted. It's possible that a Sagarika II, which may already be in development, is designed to fit the Arihant silos.
The new Indian SSN was long referred to as the ATV (Advanced Technology Vessel) class. The ATV project was kept secret. One reason for the secrecy was that so much of the ATV project involved developing a compact, light water reactor technology that would fit in a submarine. This 83 MW reactor makes the Arihant underpowered by the standards of other SSNs, and the Indians give the Arihant's top speed as 55 kilometers an hour.
Once the Arihant class design is proven, a slightly larger version will be built as a class of three SSBN (ballistic missile carrying sub). This was how everyone else did it, including the Chinese and Americans. Get an SSN operational, then modify the design to include some SLBM launch tubes. India also plans to build six SSNs based on the Arihant. All ten subs are part of a program that will eventually cost over $10 billion.
Early next year, India will take possession of a leased (for ten years) Russian Akula II nuclear sub. The Akula II is normally armed with cruise missiles, in its four larger (530mm) torpedo tubes. Since these have a range of 3,000 kilometers, they cannot be sold to India because of the Missile Technology Control Regim treaty Russia signed. Instead, the Indians will use the shorter (300 kilometers) range Klub missile. The Akula II also has four normal sized torpedo tubes. The Akula II boat will mainly serve to train Indian sailors who will operate the three SSBNs (nuclear powered subs carrying SLBMs) and six SSNs (torpedo armed attack boats.)