For movement:
Because moving diagonally covers more ground, you count that movement differently. The first square of diagonal movement you make in a turn counts as 5 feet, but the second counts as 10 feet, and your count thereafter alternates between the two. For example, as you move across 4 squares diagonally, you would count 5 feet, then 10, then 5, and then 10, for a total of 30 feet. You track your total diagonal movement across all your movement during your turn, but reset your count at the end of your turn.
For reach:
Reach is how far you can physically reach with your body or a weapon. Melee Strikes rely on reach. Your reach also creates an area around your space where other creatures could trigger your reactions. Your reach is typically 5 feet, but weapons with the reach trait can extend this. Larger creatures can have greater reach; for instance, an ogre has a 10-foot reach. Unlike with measuring most distances, 10- foot reach can reach 2 squares diagonally. Reach greater than 10 feet is measured normally; 20-foot reach can reach 3 squares diagonally, 25-foot reach can reach 4, and so on.
And is it anything like magnets?
As a tip, most grids I've seen are exactly one inch a square. If you don't feel like counting diagnols on a long move action you can just use some measuring tape to check the distance. This has saved me a lot of agony.
Yes