Well-drained soil for the plants is quite sandy and water goes through it easily. Poorly-drained soil would often mean clay-like, thick, rich soil but it retains water well that may cause roots to rot in some plants. Boggy soil is found around the ponds.
Azaleas and rhododendrons prefer acid soil and would never grow in alkaline soils so it is improtant to know what soil you've got. Go to your local garden center with a sample of soil to get an advice.
Sandy soil means the water will sip through it fast. Clay soils are rich in nutrients but may cause roots to rot for some plants that do not tolerate wet conditions. Chalky soils are alkaline, so avoid planting acidicity-loving plants. Loams are usually a mix of clay and sand, and have good amount of nutrients and a decent drainage.