Structural isomers have the exact same molecular formula but different connectivity or arrangement of atoms. This means that the contents of the isomers are exactly the same, but the way they are arranged is different. So, the main difference between structural isomers is the overall structure of the two molecules. One example of structural isomerism would be the molecules glucose and fructose. Both have the exact same molecular formula (C6H14O6), but the functional groups of glucose and fructose are arranged in a different order. So, you should always examine the structure of structural isomers rather than simply the chemical makeup.
Optical isomers are one type of stereoisomerism, which is isomerism in three dimensions. This type of isomerism means that that the two molecules have the same molecular formula and are mirror images of each other. So, the differences in optical isomers can be found by drawing the mirror image of each molecule and superimposing them. If they are not able to be superimposed, they are optical isomers. One great example of this are your right and left hands. They are exactly the same but cannot be superimposed on top of each other. Another difference between optical isomers is the way they rotate plane-polarized light. For example, two molecules that are optical isomers will bend the light in opposite directions.
Conformational isomers are another type of stereoisomerism. These types of isomers have the same molecular formula and can be converted into one another by rotating around a single bond. So, the main difference between conformational isomers is that one bond has been rotated. If rotating only one bond in the first isomer results in the second isomer, they are conformational isomers.
Isomers, specifically conformational and structural isomers, also have differences in physical properties. For example, the melting point and boiling point for isomers are usually different. This is due to the fact that the structure of one isomer may strengthen or weaken the intermolecular forces in that isomer. This directly influences the stability of the molecule and changes the melting and boiling points. So, another difference between isomers are the physical properties they display.