There is a very nice [?]description in Wikipedia[?]. There are a few different technologies and approaches but, like David said, the two terms are indeed synonyms as long as you mean whole transcriptome sequencing.
I think RNA-Seq is more wide term. RNA-Seq includes transcriptome sequencing (polyadenylated RNA), "all" RNA sequencing (include rRNA and non-coding RNA) and variant for various non-coding small RNA sequencing. The main difference in a method of probe preparation.