A mixture is defined as:
'Two or more substances, mixed but not chemically joined.'
To put this a simpler way, it is two or more substances that are present in the same container but are not joined to each other by chemical bonds.
This is shown in the diagrams below:
Shown here is a mixture of elements. An element contains only one type of atom and, because these atoms are not combined, it is a mixture of elements.
Shown here is a mixture of compounds. A compound is made of two or more elements chemically joined together. Look closely at the diagram and you will see the two different compounds but, because they are not joined to each other, it is a mixture of compounds.
Shown here is a mixture of an element and a compound. The two pink atoms combined represent the element. The atoms are the same and so this is an element. The compound is made of different atoms. Air is a mixture of elements such as nitrogen and oxygen but also contains a small amount of other gases such as carbon dioxide, and the noble gases.