The endoplasmic reticulum (ER) is a vital organelle in eukaryotic cells, playing a significant role in protein synthesis and lipid metabolism. The ER comes in two forms: rough ER, which is studded with ribosomes and involved in protein production and modification, and smooth ER, which synthesizes lipids and detoxifies certain chemicals. The ER serves as a site for processing and folding proteins, ensuring they are correctly modified before reaching their destinations. Dysfunction in the ER can lead to various diseases, emphasizing its significance in maintaining cellular function and health.