Mixtures contain at least two pure substances and can--at least in theory--be separated by physical means. Mixtures of the same substances will not necessarily contain the same proportions of those substances.
Elements and compounds have constant properties which can be described as both physical and chemical. Most of the substances we will work with and study during the course are compounds.
Compounds vary a lot. Even compounds containing the same elements like water (H2O) and hydrogen peroxide (H2O2) have very different chemical and physical properties. So it can't be just the kinds of elements present in compounds that determines their properties.
Compounds can be roughly divided into two very large categories based on the way the elements are put together in them.