A bearish divergence occurs when prices continue to form higher highs (typical in a bull market) while your oscillator (f.e. RSI) is forming significantly lower highs (indicating weakness in the trend.)
Before getting too excited about divergences as great trading signals, it must be noted that divergences are misleading in a strong trend.
A positive reversal forms when RSI forges a lower low and the security forms a higher low. This lower low is not at oversold levels, but usually somewhere between 30 and 50.
A negative reversal is the opposite of a positive reversal. RSI forms a higher high, but the security forms a lower high. Again, the higher high is usually just below overbought levels in the 50-70 area.