Ahti Heinla (born 2 May 1972) is an Estonian computer programmer and businessman. He is one of the developers of Skype. He was also an organizer of Let's Do It 2008, a civic action with 50,000 volunteers participating in cleaning up the countryside of Estonia in one day.

Priit Kasesalu (born 10 April 1972) is an Estonian programmer and software developer best known for his participation in the development of Kazaa, Skype and, most recently, Joost. He currently works for Ambient Sound Investments and lives in Tallinn, Estonia.

He started off as programmer for a local hardware manufacturer in 1986 with his class-mate Jaan Tallinn. The company was assembling 8-bit computers for use in public schools.

Jaan Tallinn is partner and co-founder of the development company Bluemoon which created the game SkyRoads.

Jaan Tallinn graduated from the University of Tartu in 1996 with a BSc in Theoretical Physics with a thesis that considered travelling interstellar distances using warps in space-time.

Skype is a telecommunications application that specializes in providing video chat and voice calls between computers, tablets, mobile devices, the Xbox One console, and smartwatches over the Internet. Skype also provides instant messaging services. Users may transmit text, video, audio and images. Skype allows video conference calls.

First released in August 2003, Skype was created by the Swede Niklas Zennström and the Dane Janus Friis, in cooperation with Ahti Heinla, Priit Kasesalu, Jaan Tallinn, and Toivo Annus, Estonians who developed the peer-to-peer backend that was also used in the music-sharing application Kazaa. 

Skype allows users to communicate over the Internet by voice, using a microphone, by video using a webcam, and by instant messaging. Skype implements a freemium business model with Skype-to-Skype calls being free of charge, while calls to landline telephones and mobile phones (over traditional telephone networks) are charged via a debit-based user account system called Skype Credit. 

In March 2020, Skype was used by 100 million people on a monthly basis and by 40 million people on a daily basis. That was a 70% increase in the number of daily users from the previous month, due to the COVID-19 pandemic. 

According to en.wikipedia