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A solution is a homogeneous mixture where a solute dissolves in a solvent. Solubility depends on intermolecular forces ('like dissolves like'), temperature, and pressure. Polar solutes dissolve in polar solvents; nonpolar in nonpolar. Dissolution is an equilibrium process; a saturated solution contains the maximum dissolved solute at that temperature.
A solution forms when one substance (the solute) disperses uniformly throughout another (the solvent) at the molecular level. Unlike a suspension or colloid, you cannot see the individual solute particles — the mixture is homogeneous. The most familiar example is salt dissolving in water, but solutions also include gases dissolved in liquids (carbon dioxide in soda), liquids in liquids (ethanol in water), and even solids in solids (metal alloys like bronze).
The central principle governing solubility is "like dissolves like," which builds directly on your understanding of intermolecular forces. When a solute's intermolecular forces are similar in type and strength to those of the solvent, the solute-solvent interactions can effectively replace the solute-solute and solvent-solvent interactions that must be broken during dissolution. Table salt (NaCl) dissolves readily in water because water's strong dipole can stabilize the separated Na⁺ and Cl⁻ ions through ion-dipole forces. Oil does not dissolve in water because oil molecules interact through weak London dispersion forces, and these cannot compete with the strong hydrogen bonds that water molecules form with each other — water molecules would rather stay bonded to each other than accommodate nonpolar intruders.
Dissolution is an equilibrium process. When you first add a solid solute to a solvent, molecules leave the solid surface and enter solution. As the concentration of dissolved solute increases, some dissolved molecules return to the solid. Eventually, the rate of dissolution equals the rate of recrystallization, and the solution is saturated — it holds the maximum amount of solute at that temperature. An unsaturated solution contains less than this maximum and can dissolve more. A supersaturated solution temporarily holds more dissolved solute than equilibrium allows, and a small disturbance (a seed crystal, a scratch on the glass) can trigger rapid crystallization.
Temperature and pressure also affect solubility in predictable ways. For most solid solutes in liquid solvents, solubility increases with temperature — the extra thermal energy helps overcome the lattice forces holding the solid together. For gases dissolved in liquids, the pattern reverses: solubility decreases with temperature (which is why a warm soda goes flat faster) and increases with pressure, as described by Henry's law. These relationships matter in contexts from cooking (why you degas water by boiling it) to deep-sea diving (why ascending too quickly causes nitrogen bubbles to form in the blood).