ISI stands for the Indian Standards Institute, a body set up when India gained Independence to create standards needed for orderly commercial growth and maintaining quality in industrial production. Certification for ISI mark is given by the Bureau of Indian Standards (BIS), the National Standards Body of India, involved in the development of technical standards (popularly known as Indian Standards), product quality.
An ISI Mark is't mandatory (unless you are making an item that falls on the mandatory list - long list to mention here that includes edible items, cement, certain home appliances, certain car parts, steel products, etc.)there are 16 broad categories, including textiles, packaged water, food, automobile components, plastic products and electronics for which BIS has laid down standards. If anyone wants to add a new category to the current list, they can apply for it. There are 19,000 standards formulated for products across the 16 categories.
The broad areas under certification are:
1- Textiles
2- Chemicals and Pesticides
3- Rubber and Plastic products
4- Cement and concrete products
5- Building materials
6- Pumping, irrigation, drainage and sewage equipment
7- Pipes and fittings for water supply
8- Basic metals and fabricated metal products
9- Machinery and equipment
10- Electrical,electronics and optical equipment
11- Automotive components
12- Agriculture, food and tobaccos
13-Black tea and beverages
14-Packaged drinking water and Natural mineral water
15-Leather products
16- Wood products
17- Paper and pulp products
18- Testing instruments

However, even if not mandatory for you, getting an ISI Mark is always a good idea as it exists for almost all kinds of products. Any manufacturer who feels confident enough that his product has the ability to meet the BIS standard can apply for product certification.

Having an ISI Mark distinguishes your products from other non-mark products. Considering how well known the ISI Mark is, a customer is bound to choose your products over the other non-mark products if has the ISI certification.
Products where our service can be applied to :

Simple pressure vessels
1- Electrical equipment
2- Toys
3- Lifts
4- Machinery
5- Radio and telecommunications terminal equipment
6- Gas appliances
7- Recreational craft
8- Electronic equipment
9- Personal protective equipment
10- Non-automatic weighing instruments
11- Pressure equipment
12- Active implantable medical devices
13- Medical devices
14- In vitro diagnostic
15-Cableways
16- Construction products
17- Measuring Equipment
18- Explosives for civil use
19- New hot water boilers
20- Equipment and protective systems for use in explosive atmospheres

PROCESS:
Submitting an application at the nearest BIS office. A BIS officer will then evaluate at the factory level, the capability of the manufacturer to produce goods according to the standards laid down for the category. Samples are tested at the factory and outside. If the evaluation is satisfactory and the product passes the tests, a license is granted and the manufacturer can use the coveted ISI mark.

1- The manufacturer provides test reports to BIS after it gets the product tested in the bureau's labs and gets the necessary documents certified independently. BIS is supposed to check the veracity of the reports within a month and grant a license for usage of the ISI mark.

2- While product certification is otherwise voluntary, there is, however, a list of items which for reasons of public health, safety or mass nature of consumption are mandatory certified by BIS. These include gas valves and cylinders and infant food. Manufacturers of such products can't apply for ISI mark under the voluntary scheme.