Dissolving is where a solid or liquid solute is spread or spreads itself throughout a solvent to form a mixture called a solution.
Many things can affect the ability of something to dissolve in something else. Temperature increases the rate at which molecules are moving, and so increased temperature speeds up dissolving. Reducing the temperature will also decrease the speed.
One particular solvent can only hold so much solute. You can mix a solute into a solvent to create a solution, but eventually it will reach what is called saturation point. This is where no more of the solute can be dissolved in because there's basically no more space for it.
A solution is a mixture of at least two substances, where one is dissolved in another. The substance which is dissolved should be evenly spread through the mixture. The most common solutions are liquids, but the things which are dissolved can be solid, liquid or gas.
The substance which has things dissolve in it is a solvent. Anything dissolved in it is called a solute. The process of mixing them together so that the solute is spread evenly through the solute is called dissolution or solvation, but is most commonly referred to as dissolving.
A solute is something which is dissolved or mixed in to something else. For instance, when you put sugar in your tea, the sugar is a solute.
When solutes dissolve in solvents, they spread apart so that they are evenly distributed throughout. When salts (e.g. sodium chloride) are dissolved in water, they break up into ions (i.e. charged particles) and turn the solution into something that can conduct electricity. The result is something called an electrolyte.
A solvent is something which other things are mixed into to create a solution. It is commonly a liquid. For instance, when you mixed salt into water for boiling some vegetables, you are making a solution out of salt (the solute) and water (the solvent).
Solvents can be solids, liquids or gases, but if the solvent is a gas then the only things which could dissolve in it will be gases (anything else would just fall to the bottom because of gravity and could not be spread evenly through the gas). Solutions will tend to be in the same state as the solvent (e.g. water is a liquid, and a solution of water and sugar will also be a liquid). However, certain things might change - dissolving salt in a liquid changes the melting and boiling point. Because salt lowers the melting point of water, it is useful in helping ice to melt.