Any wood branches, logs, or even roots continuously exposed to moving water of a stream or river or ocean tides can become driftwood. The bark wears off and only the unique structure of the wood remains. They can be harvested directly from the source or purchased especially for landscaping from garden centers and other sources including the internet.
Driftwood in landscaping provides two benefits. First, smaller pieces with more complex shapes are frequently used as accents in the middle of a garden bed. Imagine a piece of driftwood with many spikes and swirls peeking out of an attractive groundcover. Several pieces can even be lashed together to form rough fences or plant stands. Driftwood offers a realistic accent near backyard ponds and watercourses as well.
The second benefit for driftwood in landscaping pertains to adding structure to the landscaping look. Large pieces can be used to edge gardens, support climbing plants, or even as planters themselves. An old hollowed out driftwood log can support many plants. Even smaller pieces of driftwood can have small holes drilled in them where sedums and other small plants will thrive. Unless there is a lot of room for soil, choose plants that do not need a lot of moisture.
To use driftwood in landscaping, clean it off thoroughly first to make it as attractive as possible. If desired, the wood can be treated with some form of preservative so it does not continue to rot or wear away once you have it in the garden.