As of the 1.2 update (November 2013), it's very difficult to compile a full list of the differences, largely because the number of items, enemies, and equipment in the game as a whole is incredibly vast. However, here's a high-level overview:
The PC version is the "flagship" version of Terraria, and is the version that receives the most frequent updates. At the present time, it also generally receives new content first.
The console version has some exclusive items, exclusive enemies, and an exclusive boss, as partially listed in Celta's answer.
At its original release, the console version of the game was based on PC version 1.1, with a handful of exclusive items, as previously mentioned. As of November 5, 2013, a portion of the content from the PC version's 1.2 update has been added to the console version, with the rest of the 1.2 content slated to be added in the coming months. If this pattern continues, it's reasonable to expect that the console versions will receive most of the future PC version content, albeit delayed slightly.
The mobile version features some of the exclusive enemies and items from the console versions, but has some additional limitations: Hardmode is not present, only Small-sized worlds are supported, the monster spawn rate is lower, and so on. As of November 2013, multiplayer is supported, but over Wi-Fi only. A handful of additional exclusive items are present, as well, but they're mostly cosmetic.
Generally, while most of the core gameplay is present in the mobile version, it is noticeably barer in content and functionality than its console and PC brethren. However, the price is lower to match.