Canon EOS Rebel T3 vs T3i

Hi--My daughter in law is considering getting a DSLR, and I offered to help her narrow down her choices. I am puzzled by the meaning of the "i" after these models. I know the T3 seems to have gotten a somewhat mediocre review here on Dpreview ("decent image quality"), at least compared to the T3i, and I am not sure why. As the T3 is a very good price for a DSLR, is there something I should know about why one might avoid it? Amazon's consumer reviews are very positive about it, though. Is it the plastic body?

Thanks.

--
Liz