Rocks are made up of one mineral, or a mixture of minerals.
Minerals are made up of elements, which are simple substances
which cannot be broken down into any other substance. Sometimes
minerals are made of only one element, but most minerals are
made up of two or more elements. Many minerals are made up of a
large number of elements.

Minerals are often found as shapeless lumps. These pieces are
called "massive". Some minerals can also form special shapes,
which can help you to identify them. 

The mineral is said to "grow" when the elements that the mineral
is made of gets built up in layers on the mineral's outside
surface. Minerals grow into all kinds of rough shapes in the
spaces between other minerals around them. If a mineral can grow
freely, such as on the sea bed, or in the hole of a rock, it may
form beautiful crystals. 

Minerals also form other shapes which are not crystals. The
mineral smithsonite forms rounded crusts on rocks and minerals.
Pyrite may form smoothly rounded lumps called nodules.

If you find a mineral with perfectly flat surfaces, it is likely
a crystal. Some minerals break cleanly when hit, leaving pieces
with smooth surfaces that look like crystals.

These clean breaks are called "cleavages". Each mineral tends to
break or cleave more easily in some directions than in others.
The way it breaks is called its cleavage pattern. Often you can
identify minerals by a combination of their crystal shape and
their cleavage pattern.

Related Internet resources:
http://toyscollectibles.allinfo-about.com/features/minerals.html