Originally Posted by v1nce

Ti = Titanium (Alu = Aluminium). Most 'serious' racing saddles have a Titanium undercarriage/construction to make them very light. To be honest i am no fan of these saddles. Sure they are light and flashy (theft!) but they also tend to very expensive and not very comfortable (IMO!). As always i recommend a Brooks Saddle. Heavy but very comfortable (important on small wheels) and durable. You have racing versions of these as well But at the end of the day it all depends on your preferences and what you intend to do with the bike.

Carbon bars. Handlebars made from Carbon Fiber. This is stiff and extremely light though again you pay for that. Racing bikes almost always have 'drop' carbon bars.

The stem is the bit/tube that attaches your handlebars to the frame. Racing bikes tend to have a short stem so you have an air air efficient posture.

Clipless pedals, pedals that don't use a clip (metal bit) to keep your foot lodged/attached to pedal (this gives a significant improvement in pedalling strength and stamina, though it might not be worthwhile in the city/for short distances). Clipless pedals that require special shoes and/or cleats are made by a variety of companies. The best IMO are the ones that can be used without and without the cleat (on side is for normal shoes one for the shoes with cleats). This offers efficiency for longer trips and ease of use and versatility for short ones.

I really recommend Tioga Comp Pool tires. Fast as hell, comfortable and pretty good in the puncture department too. Time tested. Check my site for more details. Inner tubes don't matter all that much i think..?

There are many worthwhile modifications that can be made to any bike and many that are a waste of cash and time. Like always i suggest checking Sheldon Brown's site and MTB review and Road bike review. That way you have the benefit of knowledge and experience of many people before you spend.