What's the tablespoon holding? Butter or oregano? Sugar or salt? Molten lead or feathers?
Grams are a unit of mass (or weight) and a tablespoon is a unit of volume. Therefore it depends entirely on what you are measuring -- it depends on the density of the substance. A tablespoon of lead will be much heavier than a tablespoon of feathers (although a gram of lead and a gram of feathers weigh exactly the same!). Similarly, a tablespoon of butter weighs a different number of grams than a tablespoon of salt.
See the Web Links for a handy automatic conversion calculator. Just enter the food item, the units you have, and the units you want... and presto!
For instance (for US tablespoon):
1 tablespoon of water weighs 14.79 grams
1 tablespoon of table salt weighs 18.25 grams
1 tablespoon of butter weighs 14.19 grams
1 tablespoon of oil (canola or soy) weighs 13.62 grams
1 tablespoon of sugar (granulated) weighs 12.5 grams
1 tablespoon of sugar (brown) weighs 13.8 grams (packed)
Some other useful conversions:
16 tablespoons = 1 cup
3 teaspoons = 1 tablespoon
28.35 grams = 1 dry ounce
1 US tablespoon = 14.7867648 ml