Isomers are molecules with the same chemical make-up but different strucural make-up. It's like a puzzle with the pieces but arranged differently. Glucose and fructose are isomers of each other. They both have the same chemical formula (C6H12O6), but yet they are arranged different structurally. Observe:
H H H OH H H
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H - C - C - C - C - C - C = 0
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OH OH OH H OH
H H H OH H
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H - C - C - C - C - C - C - OH
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OH OH OH H 0 H
Notice how they are very similar - exactly the same amount of atoms, but the atoms are arranged differently. This is a small but crucial differrence. For example, they each taste quite different and interact differently with different enzymes, despite their chemical similarity. In laymen's terms, isomers are simply molecules with identical formulas but different shapes.
KEY: C = Carbon atom, H = Hydrogen atom, O = oxygen atom.
-- and | indicate single covalent bonds.
= and || inidicate double bonds.