solute and solvent
Think of making coffee. The hot water is the host, and the coffee grounds give up their flavor into it. In a solution there are these two roles: the solvent is the host that does the dissolving (the water), and the solute is the guest that gets dissolved (the coffee's soluble compounds). The solute disappears into the solvent and spreads evenly through it.
The usual rule of thumb is simple: whichever substance is present in the greater amount is the solvent, and the smaller component is the solute. When you stir salt into water, water is the solvent and salt the solute. The labels are about roles, not about which is more important — and in a few cases (two liquids mixed in similar amounts) the distinction blurs and either name can fit.
Why split the names at all? Because they behave differently. The solvent sets the basic nature of the mixture — a water-based (aqueous) solution behaves very differently from one made with oil or alcohol. Much of solution chemistry, from how much will dissolve to how fast a reaction runs, depends on the partnership between a particular solute and its chosen solvent.
In a glass of cola, water is the solvent; the sugar, carbon dioxide, and flavorings are the solutes dissolved in it.
Solvent = the host doing the dissolving; solute = the guest being dissolved.
"Aqueous," written (aq) in equations, simply means the solvent is water — by far the most common solvent in chemistry and biology. "Like dissolves like": polar solvents such as water dissolve polar and ionic solutes, while non-polar solvents dissolve oily, non-polar ones.