Kangkong (Tagalog) or tangkong (Aklanon) is referred to in English as swamp cabbage or water spinach although it's not really cabbage or spinach. It's rather ubiquitous in the Philippines, it grows everywhere, in ditches, in ponds, on dry land, or in cultivation.
The vegetable can be stir-fried, steamed or sauteed with other ingredients. The stems will take longer to cook so cook those first then add the greens.
This vegetable does not store well so it should be used as soon as you purchase or pick it. You can store in the refrigerator overnight but it is not ideal.
If you don't have kangkong then you can use equal amounts of:
Kangkong has a mild, not bitter flavor. The leaves are harvested before the plants flower to preserve the fresh, sweet taste. In Vietnam the green is eaten raw in salad or added to soup. In other parts of Asia the greens are lightly cooked with a savory or spicy paste made with chiles. The stems take a bit longer to cook so they should be added first then the leaves only take a few seconds to heat and wilt.
This is not a common vegetable in the U.S. Look for it in ethnic markets or buy kangkong seed and grow your own.