The endoplasmic reticulum (ER) is a network of membranes found within eukaryotic cells. It plays a crucial role in the synthesis, folding, modification, and transport of proteins and lipids. The ER is divided into two types: rough ER, which is studded with ribosomes and primarily involved in protein production, and smooth ER, which is involved in lipid synthesis and detoxification processes. The endoplasmic reticulum is connected to the nuclear envelope and works closely with other organelles, such as the Golgi apparatus, to ensure that proteins are properly processed and delivered to their destinations. This organelle is essential for maintaining cellular function and overall health.