Thinners and reducers in automotive paints are both solvents used to thin paint. These additives make the paint flow better so the result is even, professional coats. While the purpose of the two is basically the same, thinners and reducers are used on very different paints. Using the wrong one could ruin your paint.
The main difference between thinners and reducers is in the paint type that is being applied. Thinners are for lacquer-based paints. Reducers are used for urethane-based paints. The two solvents are not interchangeable. For example, if the paint is an enamel-based product, do not use a thinner, but rather a reducer.
Each brand of automotive paint will come with instructions on which solvent to use and what the mixing requirements are for the paint and solvent. It is very important that you follow the directions from the manufacturer exactly to get the best results. The paint will tell you explicitly whether a thinner or a reducer should be used to thin the paint.