If isomers differ by bond connectivity (i.e. acetone and cyclopropanol) they are referred to as structural isomers. That is, their chemical structure is different.
Another type of isomers, called stereoisomers, are much more subtle. Stereoisomers are isomers that have the exact same bonds between all the atoms, however the arrangement of the atoms in 3D space is different.
The primary source of stereoisomerism is what happens when a carbon atom has four different substituents bonded to it. When this happens the molecule is comparable to your left and right hands. While your hands are mirror images of each other, you can’t superimpose them (you can’t stack them on top of each other) without having your thumbs stick out on each side.
The same thing happens with stereoisomers like this. Two stereoisomers like this cannot be superimposed without having a group stick out at either end. See if you can visualize that with the alanine stereoisomers in the first photo. Because those two stereoisomers of alanine differ in the same way your hands differ from each other we say that those molecules are ‘handed’ using the fancy greek equivalent ‘chiral.’
If you see any molecule that has four different groups bonded to a single carbon atom, you know that that molecule has an identical isomer with different chirality. Remember that the bonds are identical in both molecules, they are just oriented in 3D space differently.