It's an acquired taste, Christina, sort of earthy spinach-like. You can cook the leaf part and the ribs separately like, say, spinach and asparagus. I can't say I like it very much and my OH can't stand the taste, but, no doubt, others will sing its praises. If you want to try it, I would start with a small patch, in case it's not to your liking!
If you like the taste of soil, you'll like chard Like Monika I find it very unpleasant. The very young leaves are acceptable in a mixed salad, but cooked it is a mixture of fibrous and slimy, with that oppressive earthy taste,
Cred air o bob deg a glywi, a thi a gei rywfaint bach o wir (hen ddihareb Gymraeg) Believe one tenth of what you hear, and you will get some little truth (old Welsh proverb)
I grow the white stemmed, Swiss Chard. As noted above, the leaves can be a bit earthy and bitter - but the broad white stems, braised in a little butter and stock, then served with grated Parmesan (or similar cheese) a small queeze of lemon and a dusting of parsley - Mmmmmmm.
Please also see my post some time back for Swiss Chard and green bean soup.
I agree with the comments on the flavour of the red stemmed and highly coloured chard, but I find the white stemmed variety "Silver Leaf 2" from Mr. Fothergill seeds much pleasanter. The green can be substituted for spinach or any other greens (nice with a bit of melted butter and black pepper or nutmeg) and the stems chopped and used in stews or stir fries. If you cover it with a cloche it will keep producing most of the winter, have a little rest and then start up again in spring, so it is a very useful winter veg.
When I grew the Bright Lights mix of colours they looked beautiful and the ones with pale pink stems had an acceptable flavour too.