Opalescent colors are what you would also call iredescent. When you hold the color at different angles, it shows several soft colors.
Shimmer polishes are the ones that look "sparkly", or "glittery".
Frost is similar to Shimmer, put frost is softer, it's more of a "pearly" shimmer, versus a
"glittery" shimmer.
Cremes are flat colors that have NO glitter, shimmer, or frost. They can be brown, red, orange or any color, but have no sparkle in them.
Sheer polishes are what you'd use with a french, or for a nude/natural effect.
Hope I didn't confuse you. Great question though, different polishes can be hard to figure out!