Tarp shelters are very versatile. Where bugs are few, they can be used with a groundsheet in place of a tent. In still air, a tarp overhead reflects body heat back to keep you several degrees warmer. They can be used to build a sheltered kitchen area away from your sleeping quarters, or can quickly erected as windbreaks or rain shelters on lunch stops. Tarp shelters pitched above a bivy sack make waiting out bad weather a less claustrophobic experience.
Basic square or rectangular tarps are the most adaptable. They can be set up as a flat roof, or as an A-frame, lean-to, or pyramid to block wind. Look for lots of guy points to keep your options open. More tailored and scalloped wing tarps are easier to set up so they're stable in wind and don't pool rain, but they're generally intended for one pre-defined set-up. However, you can still raise, lower, or tilt these tarps for additional wind or rain protection. Shaped tarps that are intended as your primary shelter have a high point that can be suspended or propped-up with a pole. For wind-proofness they need to be staked out at the bottom, ideally with a gap between the tarp and the ground for ventilation.
In rain, be sure your tarp has a high point for water to drain from: a ridgeline rope, central pole, or a slanting ridgeline that raises one edge higher than the rest of the tarp. Also be sure the rain has clear channels to run off the tarp edges; if necessary, rig a short line to pull down the centre of one tarp edge. Deeply pooled water can collapse the tarp, or tear its fabric or seams.
The ideal "poles" for rigging tarps are trees: they're sturdy and you don't have to carry them. Some tarps come with poles, for tree-free locations, you can also use your trekking poles, or improvise with, driftwood, skis, ski poles, or paddles.