I guess when thinking about it all the trades are basically between people to people. So emotion and psyche plays a big part in prices. Although I have also heard now that computers can execute trades much faster and more accurate that trades are often between computer to computer. Can this still result in the same way as people trading with people?
Trading between computers is very fast, if you mean algorythmic trading then it's microseconds, for tiny amounts.
I'm pretty sure they don't affect the price at all, they just look for tiny imbalances in price that are very fast, buying and selling almost instantly for tiny fractions.
A predictable, algorithm-based trading system is even more open to manipulation in this manner.