What is the difference between building sand and plaster sand
It's all about particle size, which is classified using the Wentworth scale or The Krumbein phi (φ) scale.
Sharp Sand
Sharp Sand, also known as Concrete Sand is a coarse sand with larger particles. This type of sand is typically used in concrete.

φ scale - 1 to 0
Size range - 1/2 to 1 mm (0.020–0.039 in)
Builder's Sand
Builder's Sand, also known as Plasterer's, Mason, or Bricklayer's Sand is a finer grade sand with smaller particles. It's often mixed with water and cement to make mortar for laying bricks/blocks.

φ scale - 3 to 2
Size range - 125–250 µm (0.0049–0.010 in)
Jointing Sand
Jointing Sand, also known as Beach Sand is very fine sand with very small particles. This type of sand is used for grouting joints in paving or patio slabs.

φ scale - 4 to 3
Size range - 62.5–125 µm (0.0025–0.0049 in)