What is the difference between compounds and mixtures?
Compounds are formed by the creation of chemical bonds between two substances, whilst mixtures do not form via the creation of chemical bonds.
For example, you can take some finely divided iron (iron filings) and mix them together with powdered sulphur, stir them up well, and you end up with a "mixture" of iron and sulphur. If you look closely you will be able to see the small pieces of iron sitting within the sulphur powder. Take a magnet, move it around in the mixture and you will be able to separate out the iron, leaving the sulphur behind. That's a mixture.
If, however, you take iron and heat it in the presense of sulphur you create chemical bonds between the iron and sulphur, forming iron (II) sulphide. This is a black, water-insoluble solid. It has no similarity to either iron, nor sulphur, and you cannot separate it back into iron and sulphur except by means of chemical reactions. That is a compound.