A Trekker wears a starfleet uniform to a convention because it's fun.
A Trekkie wears a starfleet uniform to a convention because s/he has heard that it is in style at the academy.
A Trekker has a Starfleet Academy window sticker on his car.
A Trekkie is cramming for the entrance exams.
A Trekker meets Marina Sirtis/Gates McFadden at a convention, tells
her how pretty he thinks she is, that it is too bad she is married or he
would ask her out.
A Trekkie meets Deanna Troi/Dr. Crusher at a convention, tells her how pretty he thinks she is, and asks her if she is still seeing Riker (Picard, some alien patient, et al).
A Trekker loves watching the show, nitpicking and discussing it with
A Trekkie loves watching those documentaries filmed aboard the Enterprise.
A Trekker thinks Wil Wheaton was a lucky kid who got to play a kid on
A Trekkie thinks that Wesley Crusher was a lucky kid who got to sit on the bridge.
A Trekker thinks that it is a shame that the show is coming to an end.
A Trekkie thinks that it is a shame that the crew is being reassigned and the Enterprise is being decomissioned.
A Trekker knows that there are gaping holes in the technology, but ignores
them and enjoys the show.
A Trekkie can't wait for the price to come down on those home food replicator units.
A Trekker buys pips for the rank s/he wants to be.
A Trekkie wonders why he is constantly passed over for promotion.
A Trekker tells his/her new girl/boyfriend that s/he really likes Star
A Trekkie's new girl/boyfriend is an underclassman at the academy.