Here's a breakdown:

* Aquifers are underground layers of rock or sediment that can hold and transmit significant quantities of groundwater. They are typically made of permeable materials like sand, gravel, or fractured bedrock.

* Permeable materials allow water to flow through them easily.

* Impermeable layers, such as clay or shale, do not allow water to pass through and act as barriers, confining the groundwater within aquifers.

There are two main types of aquifers:

* Unconfined aquifers: These aquifers are directly connected to the surface and are recharged by precipitation.

* Confined aquifers: These aquifers are sandwiched between two impermeable layers and are under pressure.

It's important to note that not all layers of rock or sediment are aquifers. Some layers may be porous but not permeable, meaning they can hold water but don't allow it to flow easily.