CSNET stands for Computer Science Network, and was a early computer network which was created to expand access to ARPANET in the United States of America. ARPANET was a computer network which evolved to become the modern day Internet. ARPANET was government sponsored and connected academic institutions in the United States. CSNET catered to those academic institutions which could not directly gain access to ARPANET.
CSNET was initially funded by the National Science Foundation; a United States government agency which supported scientific research. Funding for CSNET only lasted three years: due to the National Science Foundation deciding to switch funding to NSFNET. NSFNET was a larger and more sophisticated network - in comparison to CSNET - and NSFNET eventually transitioned/evolved to be the modern Internet. While funding for CSNET may have been cut in 1984, it continued to operate until 1991.
CSNET was deemed to be a success, and paved the way for more elaborate and wide ranging computer networks; which used the TCP/IP protocol suite. While CSNET was initially only a network of three nodes (Purdue, Delaware and Princeton University) it eventually expanded to include over 160 institutions. The majority of the connections to CSNET were based in the US, but as CSNET expanded a greater number were international (mostly in Northern Europe).