Isomers with atoms having the same order of bonding, but different spatial arrangements are called stereoisomers. Stereoisomers which are non-superimposable mirror images are called enantiomers. Diastereomers are stereoisomers that are not mirror images.
In glycine, the carbon indicated by the arrow is achiral because the carbon has two hydrogen (two of the same substituents) attached to it. That is not the case in alanine, and thus the carbon indicated by the arrow is chiral.
A carbon which is sp3 hybridized (tetrahedral structure) with four different substituents is called a chiral carbon. If the carbon doesn't have four different substituents or say at least the carbon has two of the same substituents, then it is an achiral carbon.