When we say +1, -2. +0.5 etc. For power of spectacles, what is the unit these numbers?

Prescription . The unit is diopters. Lasek can give you better than 20/20 vision because the laser is accurate to +/-0.01 d vs +/-0.25 d for glasses or contacts.
Diopter. The plus powers are for reading and make you more nearsighted and reduce your focusing or accommodation requirement. The units are in diopters or the ability of the lens to converge light.