There are many different views, but generally speaking, SRI is a means of investing in institutions which do not harm the social or physical environment or conflict with your personal values. SRI has been around since the early 1920's and began with religious institutions that wished to avoid companies whose profits were derived from tobacco or alcohol. During the 1960's and 1970's interest peaked as people opposed the Vietnam War and Apartheid in South Africa. SRI has steadily increased from just a handful of mutual funds to over 60 funds today. Companies are screened by various values but the main screens include human rights, environment, weapons and alcohol.
Tobacco is a leading screen for virtually all socially aware investors.
Investors are attracted to SRI because they want to avoid supporting harmful causes that do not reflect their values. Social responsibility means different things to different people and your definition may not be the same as the mutual fund managers. The hardest part is trying to find a fund that reflects your objectives. For example, lets say you are looking for a fund that avoids tobacco products. A fund manager may avoid tobacco manufacturing companies but may include companies that actually sell cigarettes or paper manufacturers.