1. Filamentous: These algae form long, thread-like chains of cells, often resembling hair. Examples include Spirogyra and Oscillatoria.

2. Colonial: These algae consist of groups of individual cells that are connected and often resemble small, spherical or flat colonies. Examples include Volvox and Pandorina.

3. Unicellular: These algae are single-celled and can be free-floating or attached to surfaces. Examples include Chlamydomonas and Euglena.

4. Multicellular: These algae are complex, with many cells organized into tissues and organs. Examples include kelp (seaweeds) and some red algae.

Keep in mind that this is just a general overview, and there are many variations and exceptions within each category. Algae exhibit incredible diversity in their forms and structures!