Seaweeds are multicellular forms of algae. Seaweeds constitute a significant biotic component of various ecosystems and form the base of marine food webs. Based on photosynthetic pigments, they can be categorized into three classes: Rhodophyta (red), Phaeophyceae or Ochrophyta (brown), and Chlorophyta (green). Red seaweed has the highest protein content, comparable in quantitative terms to legumes at 30–40% dry matter, and its polysaccharides are sulfate galactans, such as agar and carrageenan. However, brown seaweeds are characterized by polysaccharides with higher antioxidant capabilities than red and green seaweeds because they contain more polyphenols and sulfates. Brown seaweeds are usually large and range from the giant kelp, which is often 20 m long. Green seaweeds are the most nutrient-dense species and excellent sources of micronutrients. Factors influencing seaweed distributions are deeply discussed: a physical (temperature, substrate, light quality and quantity, tidal dynamics, winds, and storms); b—chemical (pH, salinity, nutrients, gases, and pollution); and c—biological (herbivores, microbes, epiphytes, endophytes, symbionts, parasites, and diseases). The diversity patterns in macroalgae communities on a global scale are highlighted with special reference to many case studies in Egypt.