What will happen with my beer if fermentation takes place either at a too hot or too cold temperature? What will be the effects on the flavour?
Each yeast strain has an ideal temperature range to limit bad esters and phenols.
Too hot can give the yeast too much room to play, while this is great for yeast health and reproduction they will produce esters that may not be desired, also larger molecule alcohols (fusels). So the temp is limited to isolate them to a specific metabolism that produces more favorable esters and simpler alcohols.
Too cold If the yeast is too cold thier reproduction and feeding metabolism is retarded. Resulting in not enough yeast growth to start or finish a fermentation, leaving the beer underattenuated and sweet. The yeast that did grow will be stressed to do all the work of a larger cell count, and may also produce bad esters.
Cell Count Pitching the proper amount of yeast is just as critical as temp control. As it's the growth phase that produce the most esters.
Some notable examples of how yeast esters are played with. Traditional wheat beers call for a bubblegum ester that is achieved by underpitching compared to other ales. Many Belgian yeasts are very happy making desired esters at high temperature compared to other ales. Lagers get thier clean profile from cold fermentation but a much larger pitch compared to an ale.