Rangers: Fast deploy able light infantry battalions. Specialize in seizing airfields and raiding in general. Can be deployed by either plane (parachute static line or helicopter).
US Army Special Forces (Green Berets): Specialize in Foreign Internal Defense, training local forces usually irregular type/guerrilla forces. Can be tasked to close combat missions, largest thing they provide is access to US airpower. Deployed generally as 12 man ODA teams. They are extreme specialists in their area of function and can take a group of fuck ups and make them into decent soldiers.
SEALS: Can be deployed anywhere and anyhow. HALO jumps, aircraft insertions, boat and SCUBA. They excel at close quarters combat, hostage rescue and raiding on a smaller scale than what would be tasked to Rangers. Also involved in long range patrols, surveillance missions, etc. Also, excel at being Hollywood heroes and making sweet movies.
Delta: An offshoot of the ODA teams. They are close combat specialists and are house to house/CQB fight experts. Deploy in smaller teams than a lot of others, are much more out of the limelight then the SEALS.
There are also Marine SF teams, and Air Force Pararescue and Air Combat Controllers.