During low tide, you may be subject to exposed rocks or reefs that weren't there six hours before. Waves tend to be steeper and break farther from shore during low tide, but this isn't always the case. We're speaking in general terms here, but high tide usually means mushier, slower-breaking waves. High tide can also cause backwash, which is caused when waves bounce off rocks or sandbars on shore and ricochet back to sea. During high tides, you will notice that shorebreaks become more intense.
Most breaks have a preferable tide.
To find out when to go to your local break, grab a free tide book from your local surf shop. In addition to a tide book, it's always a good idea to carry along a weather radio, available at any RadioShack or marine-supply store. The weather radio will keep you updated on high-surf advisories, buoy and wind readings and swell forecasts. Like death and taxes, it's a sad but necessary part of life. A blackball flag, characterized by the black circle surrounded by a yellow background, means that hard surfboards are not allowed in the break.