One thing must be said: if you feel a 'clear' sensation of bitterness when tasting a wine this means that the wine has a fault, probably due to a bacterial infection.
If what you feel on the bottom of the tongue - more on the lining of the mouth than on the tastebuds - is something 'close to' bitterness this is called astringency. Astringency causes the gums to contract, gives an impression of dryness and roughness on the tongue. It is due to tannins, so it is easier to spot in red wines - which the wine takes from the stalks, skins and seeds during maceration It is noticeable in young wines and diminishes gradually with time.
The sensation of bitterness which is sometimes found in white wines, however, may be due to a fault (the presence of aldehydes and ethyl acetate) and in other situations (certainly more pleasant to the palate) it may be characteristic of the grape variety (odour of nuts like bitter almond or green walnut).