A custom-built PC is a personal computer that is assembled from individual components according to the user's specific needs and preferences. This allows for greater flexibility in choosing parts such as the CPU, GPU, motherboard, and RAM, enabling users to optimize performance for tasks like gaming, video editing, or general use. Building a custom PC can be more cost-effective than buying a pre-built system, as users can select high-quality parts that fit their budget. Additionally, it offers the opportunity for upgrades and repairs, making it a popular choice among tech enthusiasts and gamers who want to tailor their machines to their unique requirements.