1. Time-Based e.g. every 5, 10, 15, 30 minutes
2. Tick-Based e.g. every 800, 2000 trades
3. Volume-Based e.g. every 100 contracts
4. Price-Range-Based e.g. every 10, 20, 50 pips

I find price-range-based charts to be superior to the first three.  The bars look like standard chart bars with an open, high, low and close with the following characteristics:

1. The high to low range of each bar is constant. 
2. A new bar is formed only when price exceeds the fixed range of the current bar. 
3. Each bar is the same height because the range is constant.
4. The close of a bar is always at the high or low of the bar.
5. The open of a bar is always one tick below or above the close of the preceding bar.
6. The time period covered by each bar varies.
7. All gaps are filled with inserted 'phantom' bars.

Here's a sample chart:

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