What do the different flags on the Lifeguard towers at the beach mean?

Question

Ive seen a yellow, green, and a yellow with a black ball...i don't know what they mean.

Answer 1

The green, yellow, and red flags indicate the water safety conditions - as in, the wave size so swimmers don't get into trouble.

Green:  Nothing to worry about, go swim.

Yellow: Moderate conditions, rip currents COULD form, use caution

Red:  Rough Surf, and strong currents, swimmers should not be in the water.

Yellow flag w/ black dot: Black ball conditions- means no surf boards, no paddle boards, no watercraft at all.  Just swimmers.  

Answer 2

The first answer is right, but keep in mind, not all beaches use the flag system.