SAG – Saggital – From the side. Top of head is up, face/chest are left. backside is right.
AX – Axial – From the top. Face/chest are up. Backside is down. Patient right is right & patient left is left.
COR – Coronal – From the back. Top of head is up. Patient right is right. Patient left is left. First image is front of body.

T1 – Short timing – Pulses (TR or Time of Repetition) are less than 1 second apart. Return signal (Time of Echo) is less than 30ms after pulse. This is weighted for showing proton density (PD), which means fluid and complex molecules.

T2 – Long timing – TR over 2 seconds, and TE over 80ms. This picks up lower density structures better, but rapidly flowing fluid, especially arterial blood, will already be out of frame before the echo is returned. This picks up MS better, because of the high density signal from the plaques and lesions is strongly contrasted by the low signal from the CSF.

FLAIR – Fluid Attenuated Inversion Recovery – Often a T2 image is really a high dynamic range image created from both T1 and T2 echoes. This is used to suppress the CSF signal, while still picking up the details from T1.

Signal differences
Different types of anatomy and pathology return different levels of brightness or darkness. Comparing T1, T2 and FLAIR signal strengths can determine what type of mass exists at a specific voxel (volumetric pixel). This can be compared with reasonable physiology for the area to help provide diagnosis or indication for further assessment.

Reference
An excellent technical detail of magnetic resonance systems is available at:
http://spinwarp.ucsd.edu/neuroweb/Text/br-100.htm
BASIC PRINCIPLES OF MR IMAGING by John R. Hesselink, MD, FACR