ACID transactions, characterized by Atomicity, Consistency, Isolation, and Durability, are crucial for ensuring data integrity in databases, especially in multi-user environments. These properties ensure that transactions are fully completed or not at all, maintain the database's valid state, allow concurrent transactions to operate independently, and guarantee that once transactions are committed, they are permanent. Isolation levels, which determine transaction visibility and interaction, play a vital role in balancing consistency and concurrency, with higher levels reducing conflicts but potentially impacting performance. Snapshot Isolation is highlighted as a method to maintain data consistency while reducing concurrency control overhead, offering a consistent view of the database at the start of each transaction. The text contrasts ACID with the BASE model, which is preferred in NoSQL databases for environments where availability is prioritized over immediate consistency. BASE models provide eventual consistency, which is suitable for large-scale applications where the overhead of ACID compliance is prohibitive. The choice between ACID and BASE depends on application needs, with ACID being essential for reliability-focused domains like finance and healthcare, while BASE suits high-scale, low-stakes environments.