To get those aromas you have to get them up into the back of your nose. Tannin - a very young wine will have lots of tannin, and this gradually softens, as the wine ages. It holds the wine together, it lets it develop. A wine with no tannin at all is very acceptable but it won't age. Bitterness - that's really quite a nasty taste, but it's not quite the same as tannin.
Complexity is a word used a lot.
All it really means is there's just more than one dimension for the flavour - that it's got lots of layers of flavour, and you can work through those layers, and register lots of different tastes. Sauvignon Blanc, for example, has greenish characteristics. Other words that people use for Sauvignon Blanc are nettles, grassy - some people even find a particularly ripe, sweaty shirt is close to Sauvignon Blanc. If you try and describe the difference between roast beef and roast pork, it's terribly difficult. It's the same with wine.