Strength of association. If the points have a lot of scatter above or below the trend line, then the association is not very strong. On the other hand, if the amount of scatter above or below the trend line is very small, then there is a strong association. Beware of plots with lots of white space around the data points. This is usually a sign that someone is trying to be dishonest about the strength of the relationship as portrayed by the data.
Outliers are values that are unusually far from the trend curve - i.e.
There is no formal rule to detecting outliers - use common sense. One's usual initial suspicion about any outlier is that it is a mistake, e.g. a transcription error. Every effort should be made to trace the data back to its original source and correct the value if possible. If the data value appears to be correct, then you have a bit of a quandry. Do you keep the data point in even though it doesn't follow the trend line, or do you drop the data point because it appears to be anomalous.