There are two kinds of trading that are referred to as "insider trading" or "inside dealing":
Usually illegal: Trading of a security of a company (e.g., stocks, bonds or stock options) based on material non-public information. The trader may be a corporate "insider" or someone unlawfully obtaining the non-public information (which may constitute a separate offence of spying on trade secrets).
Usually legal: Trading not based on material non-public information by "insiders" of a corporation. (legal)