I have recently finished the book "ISLAM AND THE FUTURE OF TOLERANCE" written by Sam Harris & Maajid Nawaz, this really educated me on some terms which are generally thrown around without understanding them. Here is how both authors describe the different terms (although they do disagree on the percentages of Muslims in each group).
Imagine a large circle which encompasses all Muslims, inside this circle are ever smaller concentric circles.
Conservative Muslims: Literalist Muslims within their own families and lifestyles.
Islamists: Are conservative Muslims who want political Islam imposed on society.
Jihadists: Are Islamists who use force.
Islamic Terrorists: Are Jihadists who also target non-combatants.