1. Number of cell layers:

* Simple epithelium: Consists of a single layer of cells.

* Stratified epithelium: Consists of multiple layers of cells.

2. Shape of the cells in the uppermost layer:

* Squamous: Flattened and scale-like cells.

* Cuboidal: Cube-shaped cells.

* Columnar: Tall and column-shaped cells.

Combining these two criteria, we get different types of epithelial tissues:

* Simple squamous epithelium: Single layer of flattened cells.

* Stratified squamous epithelium: Multiple layers of flattened cells.

* Simple cuboidal epithelium: Single layer of cube-shaped cells.

* Stratified cuboidal epithelium: Multiple layers of cube-shaped cells.

* Simple columnar epithelium: Single layer of tall, column-shaped cells.

* Stratified columnar epithelium: Multiple layers of tall, column-shaped cells.

* Pseudostratified columnar epithelium: Single layer of cells that appear stratified due to their nuclei being at different levels.

There are also some special types of epithelial tissues, like transitional epithelium, which can change shape depending on the organ's state.

The combination of cell layer and cell shape determines the function of the epithelial tissue. For example, simple squamous epithelium is thin and permeable, making it suitable for diffusion, while stratified squamous epithelium is protective and resistant to abrasion.