An "extremist" is someone whose ideolology is well outside the norm, and who passionately advocates for it in spite of, and in opposition to, common or normal ideology.
A "terrorist" is someone who kills innocent people in order to promote a political agenda.
These two words (and the categories they are meant to represent) are, or should be quite distinct. Obviously, one would hope, killing innocent people for a cause would not be regarded as identical to advocating for some radical ideology or other.
Unfortunately, the degree of semantic sophistication in our major communication networks is not all that it might be. The two words are often used interchangeably, by reporters and journalists who should know better. Actual terrorists are often referred to as extremists, so that legitimate extremists are often associated with terrorists.
The reason this bugs me, and the reason for this diary, is that I happen to be an extremist myself. But there is no way I would ever kill an innocent person, let alone advocate for the legitimacy of terrorism anywhere, for any reason.
Terrorism is evil -- it is always evil, no matter what it tries to accomplish. But extremism is not always evil - to the contrary, extremism can often be beneficial and necessary. It is true that most terrorists are extremists, but it is not true that most extremists are terrorists.