RIS is a tagged citation format from Research Information Systems (the Reference Manager company), now an industry-standard interchange. Two-letter tags (TY=type, AU=author, TI=title, JO=journal, etc.), one per line, records terminated by ER. Most academic databases (PubMed, Web of Science, JSTOR, Scopus) export to RIS. Reference managers all import and export it.
How to open
Zotero, Mendeley, EndNote, Papers, RefWorks, Citavi, every modern reference manager. Plain text in any editor.