Is giardiasis the same thing as traveler's diarrhea?

No. Usually travelers diarrhea i.e. Montezumas revenge is caused by e.Coli a different strain of bacteria. We all have e.Coli in our GI tract. Giardia is a parasite found in freshwater streams that can cause diarrhea, but is much less common than e.Coli infection. Special stains or endoscopy can be used to show if giardia is present.
No. The question is too limited. Giardiasis can cause traveler's diarrhea as can bacteria, viruses & parasites. The most common cause is e. Coli.