I will try to explain what i can. I find that most problems in understanding stem from ambiguous definitions of certain concepts so ill try and define some terms first.
Temperature - A measure of the kinetic energy of matter, stemming from the human sense of heat originally. The more excited and energetic a substance is the greater its temperature will be, heat will flow or conduct across a potential gradient from areas of high energy to low. These are experimentally observable properties associated with the branch of science called thermodynamics.
Solid - A state of matter in which the molecules of a substance have less kinetic energy than the energy required to overcome the attractive forces between neighbouring molecules. They fall into crystaline formations, mostly vibrating.
Liquid - A state of matter in which the molecules of a substance have less kinetic energy than the energy required to overcome intermolecular forces, but sufficiently enrgetic that they can still move about freely to some extent, flowing as a fluid. Primarily molecules in a liquid move by rotations and vibrations.
Gas - Yet another observable state of matter in which the kinetic energy of the molecules are sufficient that most molecules are free from other molecules and are free to rotate, translate and vibrate.
Now when a substance, like water, undergoes a change of state, such as freezing, the molecules loose sufficient energy ( by collisions with a lower energy substance or some other energy transffer process ) that the intermolecular forces begin to take over the kinetic forces. The molecules can no longer overcome attractive forces, and contract and fall into and ordered lattice.
In the case of water, Hydrogen bonding is strong ( one of the intermolecular forces that is observed between hydrogen and other strongly electornegative atoms ) and such that when water actually freezes the molecules align in such a way that the density of Ice is less than that of water, and so ice floats.
The most important idea i think is that of energy transfer, in order for a state of matter to change energy must flow in or out of the system. Think of it as game of snooker if you like, the pack of red balls will only "break" if you hit them hard enough with the white. The kinetic energy you need to hit them with ( as measured by Temperature ) depends on a many factors, like the weight of the white ball and if your reds are magnetically attracted to each other..etc. These properties are different for different substances and so melting and boiling points are very much charateristics of a substance and have been so used to catogorise things.
An important aspect of changes of state is that of the energy required to cause the transition. This is a key point in finding a change of state, if you heat a bath of water its temperature will rise over time as it takes in the energy. However as it reahes the boiling point the temperature will stop rising for a short period (~100C). This is the boiling point, where the energy is now being transferred to the new mode as the molecules free themselves of intermolecular forces and begin to take off as a gas, after that the tempertaure of the gas will continue to rise.
A more in depth discussion could follow about Statistical Mechanics, Boltzmann, Kinetic Theory of gasses...I could go on for a long time...must stop typing...
