This means that the price changes immediately when new information becomes available. Weak form efficiency means that current prices fully reflect all historical stock prices.
Most empirical tests indicate that the market is very close to being weak form efficient.
Semi-Strong form efficiency means that current prices fully reflect all publicly available information. This means that you should not be able to analyze all publicly available information and determine underpriced and overpriced stocks. A more moderate form of this type of market efficiency states that an abnormal return could be earned to the extent of fairly compensating the most efficient analyst for the effort spent accurately analyzing stocks to determine true mispricing. Empirical research shows that the market is close to being semi-strong efficient, but there are some small, unexplained anomalies. Strong form efficiency means that current prices fully reflect all public and private information.