Decanting wine should not be something you do to impress your guests and feel like a connoisseur; at the same time, you do not need to be a sommelier to get it right.
A racy red wine, that is young or particularly tannic, needs air to soften and release all its aromas and stratified flavours. To fasten this process, decanting a young tannic racy red wine is an optimal solution. Better it is to use a wide open-neck decanter, and let the wine rest for about 2 hours.
If opening a bottle of aged red wine, that’s a different story. First of all, the wine will have probably accumulated residues and sediments; therefore it is very important to keep the bottle upright for at least 24 hours before drinking it, which will help deposit sediments on the bottom of the bottle.
If you do not have time, or simply forgot about it, then you might want to use a decanter to eliminate those sediments; however, you should be careful because too much air might kill the few fruity and vigorous overtones still left in the wine. Therefore, use a narrow-neck decanter and aerate the wine for a shorter time.
In any case, opinions on wine decanting are many; just do your own tests and figure out what YOU believe is best. And do not forget to enjoy the wine!