1) what exactly LDAP can do in term of web base applications and others thing.
2) What the system requirement for server to use LDAP ?
3) can LDAP be standalone (testing in localhost) ?
that's all
1. LDAP is normally used for authentication, most servers contain all users and there needed info and say sites are authenticated against it. But it can be used also in other ways, LDAP is a directory based Database, so it can be used to store information the same way a directory can store files.
2. Depends what version you're using.
3. Yes LDAP can be run anywhere you have the requirements