An index is a data structure that allows for faster retrieval of data from a table based on one or more columns. Indexes increase the performance of read operations, such as select queries, by reducing the number of records that need to be scanned. However, indexes decrease the performance of write operations, such as insert, update, and delete queries, by requiring additional work to maintain the index structure and ensure its consistency with the table data. Therefore, indexes should be used judiciously and only on columns that are frequently used in queries and have high selectivity (i.e., low number of duplicate values).