In SQL, wildcard characters are special characters that allow you to perform pattern matching in queries. They are used in conjunction with the `LIKE` operator to search for strings that match a specified pattern. The two most commonly used wildcard characters are:

- `%` (percent sign): The percent sign represents zero or more characters. It can be used to match any sequence of characters in a string. For example, to find all names starting with "J", you can use the following query:

sql
SELECT * FROM table_name WHERE name LIKE 'J%';

This query will match names like "John", "Jane", "James", etc.

- `_` (underscore): The underscore represents a single character. It can be used to match any single character in a string. For example, to find all three-letter names starting with "J" and ending with "n", you can use the following query:

sql
SELECT * FROM table_name WHERE name LIKE 'J_n';

This query will match names like "Jan", "Jon", "Jen", etc.

You can also combine wildcard characters to create more complex patterns. For instance, `%an%` will match any string that contains "an" anywhere in the text.

Note that the behavior of wildcard characters may vary depending on the specific SQL database system you are using. Additionally, some database systems may have additional wildcard characters or extensions to the basic wildcard functionality. Therefore, it's always a good practice to consult the documentation of your specific database system for precise details on wildcard usage.