First, visa free travel.
Citizens from Bhutan, Nepal and Maldives can enter India without obtaining a prior visa, they can just get into a plane and travel to India and receive their visa on arrival.
Similar arrangements are common between many countries who have bi-lateral arrangements. Sri Lankans can travel to Singapore and Maldives without a visa, entry is granted on arrival, no questions asked, no backgrounds checks carried out. No entry fees are levied to the visitors either.
Second, visa on arrival
Visa on arrival requires a traveler to obtain an entry visa at the border crossing, usually upon paying a visa fee. The duration of stay is limited. India previously offered this facility to a number of counties including Indonesia, Japan and Singapore. But this system, it is learnt, is being phased out.
Third, Electronic Travel authorization (ETA)
What India has granted to Sri Lankan travelers is a facility that has been already extended to over 40 other countries and is called Tourist Visa on Arrival (TVOA) enabled through a Electronic Travel Authorization (ETA) system.
According to Indian Home Ministry website, India introduced Tourist Visa on Arrival (TVoA) enabled ETA Scheme on 27th November 2014. Initially this facility was made available to nationals of 43 countries.
The TVoA enabled with ETA would enable the prospective visitor to apply for an Indian Visa from his/her home country online without visiting the Indian Mission and also pay the visa fee online. Once approved, the applicant will receive an email authorizing him/her to travel to India and he/she can travel with a print out of this authorization.
On arrival, the visitor has to present the authorization to the immigration authorities who would then stamp the entry into the country. (Source: Ministry of Home Affairs, India).
Sources at Indian High Commission in Colombo confirmed that Sri Lankan travelers too are now entitled to obtain ETA enabled "Tourist Visa on Arrival". There are four steps involved; apply online, pay visa fee online, receive ETA online and print EAT and travel to India.
According to the Indian High Commission, the ETA process takes 4 working days and costs US$ 60 compared to Rs 2,500/=.
So if you were planning to travel to India hopping to get a "visa-on-arrival", you were sadly mistaken.