Sometimes, the way that a survey respondent answers one question will dictate whether other questions in the survey are relevant to them. For example, if someone answers that they are less than 16 years old, then a question about their driving experience might not be relevant (at least in a legal sense.) That is where skip logic comes into play.
Skip logic, or conditional branching as it is sometimes known, allows you to change the course that your respondents take through a survey based on answers they give to certain questions. You do this by creating skip rules. For example, if you create a survey and ask your respondents if they are male or female. You could create some skip rules based on this, that would then direct them to a certain page based on their response. Men for example, might bypass questions about their menstrual cycles, etc.
For more information about using skip logic in surveys, read Using Skip Logic in GroupSurveys.
Yes, skip logic is one of the key things that sets online surveys apart from paper surveys. I am still amazed when I see surveys that do not use it.