To promote "the health and well-being of gay, lesbian, bisexual and transgender persons, their families and friends" through support, education, and advocacy.
In 1973 by Jeanne Manford, who was so outraged over her gay son's beating at the hands of anti-gay protesters that she marched in the 1972 New York Pride Parade herself. The organization, originally founded as Parents FLAG, went through a series of name changes before becoming Parents, Families, and Friends of Lesbians and Gays in 1993.
PFLAG is primarily a grassroots organization, with over 200,000 members who belong to some 500 local affiliates; some affiliates are more support-oriented, helping participants better understand their lesbian and gay family members, while others are more advocacy-oriented. PFLAG does, however, have a national staff that oversees large-scale education and advocacy initiatives.
Because we still live in a very homophobic culture, PFLAG can go places that other LGBT rights organizations can't go. While PFLAG welcomes lesbian and gay members, its status as an ally-centered organization opens many doors and provides support for more policy-oriented LGBT organizations.
PFLAG has the reputation of being a support group for parents who are coming to grips with their children's sexual orientation rather than a potential venue for activists interested in pursuing a firm policy agenda. PFLAG has also been criticized for its relative lack of diversity, though the organization is taking steps to correct this problem.
As is true of any chapter-driven grassroots organization, there is going to be a great deal of variation on a local level because the people involved, and the priorities of the people involved, inevitably vary. The fact that some PFLAG chapters function primarily as support groups does not mean that your own local chapter is not activism-oriented. Besides, there's nothing wrong with support groups--the American LGBT rights movement itself began as a cluster of regional support groups during the 1950s.