NASA stands for National Aeronautics and Space Administration. It evolved from NACA, The National Advisory Committee on Aeronautics.
T-time is how time is measured prior to liftoff. When they say T minus 10 minutes, it means that, if all goes according to plans, the launch will take place in 10 minutes. The familiar 10-9-8-7-6-5-4-3-2-1 countdown is just the last few seconds of T time prior to liftoff.
Gravitational pull is the force that two objects exert on each other that attracts them together. The gravitational pull of the Earth is what keeps you from flying off into space.