Solid is one of the states of matter, alongside liquid, gas, and plasma. A solid is the lowest-energy and usually densest form of matter, characterized by molecules which are closely packed together; solids hold their shape in normal conditions, and have a fixed volume based on that shape. Solids will keep their shape rather than flow to fit the shape of the container that holds them (as liquids and gases do).
At the atomic level, atoms in a solid are usually tightly packed together, either in a regular, repeating pattern, or simply randomly. The former include crystals, complex regular formations which can be as exquisite - and valuable - as diamonds. Most solids, especially metals, are crystalline at the microscopic level. The second group, with essentially randomly arranged chains of atoms, includes such important materials as glass. Note that most commonly identified solids are actually mixtures of different substances, combined together, rather than pure crystals of a single element. Common rock, for instance, is a mixture of various metals and minerals.
SOLIDS RETAIN THEIR SHAPE
There are several characteristics common to solids. First, solids do not flow to fill the container they are placed in. To varying extents, in other words, they will retain their shape and integrity under both high and low pressure. This does not mean solids cannot change their shape: indeed, when exposed to sufficient force, all solids will eventually become deformed. (Iron and steel, for example, can be extremely strong, but a steel rod will eventually bend if pushed strongly enough).
Some solids are much more malleable than others: moreover, some are elastic (when the force which bends them ceases, the object springs back to its original shape. Objects that do not return to their original shape are said to behave plastically. Even elastic objects usually have a point beyond which elastic forces will no longer return them to their original shape (an elastic band, for example, simply snaps when pulled too tight).
ATOMS IN SOLIDS VIBRATE
In liquids and gases, the molecules or atoms which make up the substance tend to move around relatively freely. In gases, most molecules have sufficiently high thermal energy that they actually move entirely freely, potentially occupying large volumes at normal pressure. In solids, by contrast, molecules do not have energy to move around freely; instead, they remain tightly packed and vibrate within this solid structure rather than move about freely.
For this reason, solids do not gradually melt in the same way that liquids gradually evaporate. (Evaporation occurs when moving atoms moving along the edge of the liquid bounce off into the air.)
CHANGES IN STATE
All solids, if heated to a sufficient temperature and/or placed in a sufficiently low-pressure environment, will melt to become liquids (which may then evaporate to become gases). However, because solid are the lowest state of matter, cooling a solid will not produce a new state of matter. Theoretically, a solid could be cooled to such an extent that the molecules simply stopped vibrating entirely. It is theoretically impossible to achieve true "absolute zero" temperature - -273.15 degrees Celsius, -459.67 degrees Fahrenheit, or 0 degrees Kelvin - even in laboratory conditions. If it could be done, however, presumably all laws of thermodynamics would break down, resulting in some new transformation of the matter in the sample.