they distinguish between idiots and morons

srsly, don’t get either of those.
there’s no meaningful difference; one just gained faddish popularity at a slightly different time than the other one.

Celtic is more geometric. The Celtic designs usually have some kind of traditional meaning behind them and the lines on it are usually even. Tribal is more free form.

tribal is just lines with no meaning. celtic on the other hand, means something. it origins from North Europe and it actually has symbols instead of lines.
but if you still want a tribal, you should search for Maori symbols that look alike a bit

This is an example of a tribal tattoo:

http://tattoodesignspictures.com/ancient-meaning-behind-tribal-tattoos

They are usually black…. and would be used in the Pacific area.

This is an example of celtic tattoo

http://tattoosartdesignspictures.com/wp-content/uploads/2008/08/celtic-tattoos-tattoo-designs-pictures-18.jpg

Celtic incorporate lines that look like a fanciful woven knot.

Personally I like the look of Celtic designs a lot more than tribal ones.

Celtic tattoo are usually for people with a Celtic heritage (Irish, Scots or Welsh) these tattoos are often ways of expressing pride in ones heritage and a reaffirming of affinity to the indigenous artistic style of their ancestors.
http://celtic-tattoo.net/default.aspx

Tribal tattoos are more to mark a person as a member or nonmember of the local group, or a religious group, or certain spiritual beliefs and personal convictions.
The main people to get these would be the Polynesian or maouri.
Although these days they’re more commonly chosen because people like the way they look.