Alloys are a mixture of different elements. Because the sizes of the atoms of the two elements in the mixture are different in size, the layers are tightly packed and easily slide over one another. This makes alloys stronger than metals in their original states.
Some examples of alloys are :
Brass which is a mixture of copper and zinc
Solder which is a mixture of lead and iron
Nitinol which is a mixture of nickel and titanium