A carbon atom always has four bonds so we are looking at the groups attached to the carbon atom by those bonds. There are almost an infinite number of different groups that could be attached but some examples are –H, -OH, -CH3, -CH2CH3, -CH2CH2CH3, -CH2NH3. If any of these groups were attached twice to a carbon atom, then that carbon atom could not be asymmetric. Also, if a carbon atom is attached to oxygen or another carbon atom by a double bond, then, the carbon atom cannot be asymmetric.
Isomers are compounds with the same molecular formula but different molecular structures. That is, CH3CH2OH may represent CH3-O-CH3 (methyl ether) or CH3-CH2-OH (ethyl alcohol). Note that these isomers differ from each other with respect to which atoms are joined to which other atoms. Also, isomers do not have to contain an asymmetric carbon atom.
Sterioisomers: Sterioisomers are a subset of isomers. Like isomers, stereoisomers are compounds with the same molecular formula but different molecular structures.
Stereoisomers are isomers that differ from each other only in the way the atoms are oriented in space but are like one another with respect to which atoms are joined to which other atoms
Stereoisomers must contain an asymmetric carbon atom.
Stereoisomers cannot be superimposed upon one another.