Chard is available in many different varieties, some of which are distinguishable by the color of the chard's stems.
The stems range in color from red to yellow to white. You will usually see something called "rainbow chard" at the grocery store or farmer's market, which is basically a bundle of red, yellow and Swiss chard (which has white stems), rather than a specific multicolored variety.
Chard's flavor is comparable to spinach, although this depends on what cooking technique is used. It can be bitter, especially Swiss chard, but cooking tends to diminish the bitterness so that its earthy, sweet, almost beety flavor is most pronounced.
Chard also happens to be an easy vegetable to grow in your garden. You can plant it twice a year, in the spring and again in fall, and in some climates, such as here in the Pacific Northwest, it will live all year long.
You can eat the younger leaves raw, like in salads, and the more mature leaves, which are tougher, are best served cooked. Like with collard greens, it's best to remove the stems and ribs from the centers of the leaves, because they can be tough and fibrous. Some people like to cook the stems separately.