A mixture is a combination of two or more substances that are not chemically united and do not exist in fixed proportions to each other. Most natural substances are mixtures.
Here's the basics of mixtures.
Check out this website for a short explanation of the differences between elements, compounds, and mixtures.
Still uncertain? Read this answer.
Is oil and water a mixture? TEDxOilSpill speaker Lisa Margonelli explains the chemistry of oil and what happens when it spills into water.
Here's a really cool article about mixing oil and water.
Now, go do it on your own. Really observe what's happening. Ask yourself questions about mistures: Why doens't everything blend evenly? Why doesn't the oil just absorb the water? Why doesn't the oil dissolve in the water? What happens when you vigorously shake the container? Does it change when you heat / freeze it?