There are essentially two reasons why a wine should be decanted.
The first occasion is when a wine has 'thrown' a sediment. This typically occurs in a red wine that has some bottle age, perhaps 5 to 10 years +. Even though the sediment is harmless and natural (and not a fault) it needs to removed so as not to spoil the enjoyment of the wine. Drinking from a glass that has sediment in can be an unpleasant experience.
The second occasion when decanting is definitely worthwhile is when opening a young medium or full bodied red. The purpose then is to aerate the wine. Exposing it to oxygen helps to soften it and helps enhance its aromas and flavour. If in doubt it's worth decanting any young red wine prior to serving.
When to decant?
There is no definitive answer on this. However with an older wine decanting should be done around 45-60 minutes before serving. The important aromas should still be present within this time. Any longer though and these will go and the wine can then prove to be a disappointment. With younger wines anytime from 1 hour to even 24! Generally though 1 to 4 hours would be fine.
What to use?
A glass decanter is ideal and these come in numerous shapes and sizes. However, a standard jug would have the same affect. Anything where the wine has significant surface area exposed to oxygen. For this reason pulling the cork and leaving the bottle aside to breathe has little impact.
If you have any queries on this do get in touch.