Vascular plants have roots. These are an organ of the plant that lacks leaves (and nodes) that absorbs water and nutrients (typically from the soil), and anchor the plant to the soil (or host/ledge for an epiphyte). There are plenty of exceptions. For example a rhizome is actually a piece of buried stem. Similarly, roots can be aerial - especially in water logged conditions, and with epiphytes.
As well as being an essential part of most vascular plants, roots play an important role in the landscape, as they help prevent soil erosion.
There are a wide range of different root types found on various plants. For example a 'tap root' goes very deep to tap water in drought / low water table conditions. A 'stilt root' comes out of the plant's trunk a few feet up, providing support for the plant, and even allow very limited lateral movement of the plant. A tuberous root stores food or water - eg. a sweet potato.
Although barely visible, plant root systems can be very large, and as a general rule-of-thumb, typically mirror the extends of the above-ground foliage.
Use this tag for all questions about roots. For example, protecting them, pruning them, removing them, etc.