Contains - the text will can appear anywhere in the field you selected in the first box. Remember this can have some unexpected effects. For example if you set up a rule to delete any message which contains "week" in the subject field it, using the Contains rule it would also delete emails containing "weekend" in the subject field. To create a more specific rule it would be better to use the Equals option in this box. which would therefore distinguish between "week" and "weekend", but bear in mind that the Equal rule applies to the whole string in the selected field (see below).
If you use the Equals or Does not equal option remember this applies to the whole string in the selected field. Therefore if you set up a filter rule to delete any email with "week" in the message body it would only work if the only word in the entire message body was "week". Therefore as a general rule it is recommended that you do not use the Equals or Does not equal options on the message body.