They're two different manufacturers of bicycle parts, mainly gears(shifters, derailers, cassettes and chainrings/cranks, and brakes).
Both have a range of quality/price levels. For the same money, the stuff is very similar in performance. some prefer one some prefer another.
They are both great group sets but i am using Ultegra
Sure. There are two things that has to match, the number of gears, and the actuation ratio or the rear derailer. If the rear derailer is a Shimano, then it'll take a Shimano shifter no problem. If it is an SRAM derailer, then you have to figure out if it's the Shimano-compatible version or the 1:1 pull SRAM version. If it's long-pull SRAM, you'll need a long-pull SRAM shifter. Once that's sorted out, unhook shifter cable at derailer. Pull grips off bar, loosen pinch screw(usually Allen head) slide twist shifter off. Slide trigger shifter on, tighten pinch bolt. Slide grips back on. Install shifter cable. Adjust shifting. Done!
Dura-ace 7800 10 speed, but it all depends on which drivetrain (Shimano, Campy or SRAM) you have.
Specialized uses different types of derailleurs. The most common are Shimano and SRAM. Each of these have different levels to them. The better the derailleurs the more expensive the bike.
Depends. If you stay within the Shimano family RDs are usually interchangeable, but they need to have the same cage length to be able to take up tha same amount of chain slack. SRAM and Campagnolo (mostly) use another throw ratio and are usually not interchangeable with Shimano.
This will mostly depend what type of cassette body you have on the wheel. Shimano and Sram are compatible and Campgnolo is different. Those are the three most common. Secondly you should know how many speeds are on the bike as the chains and spacing on the cogs are different... I will assume 9 or 10 speed. Go into a shop and have them look at it and order you something. The last time I did this they had the part I needed in stock and all the people working there were super helpful.
Shimano's population is 7,003.
Shimano's population is 975.
SRAM owns SRAM. SRAM also now owns Rock Shox, Avid and Truvativ through aquisitions.
Shimano.
Shimano was created in 1921-02.
Shimano Nexus was created in 1995.