SQLDeveloper's primary purpose is developing inside the database, mainly in PL/SQL. It also has other features, but mainly aimed at database development or power users.
The reports part of SQL Developer is fairly basic. You can only run the reports within SQL Developer so again this is aimed at developers and power users. You can't really develop a complete application in SQL Developer
Forms and Reports are tools for developing sophisticated GUI applications. I haven't used either for years, so I don't know what they are currently capable of but they can produce very sophisticated applications.
I don't know what the future of Forms and Reports is, but the tools Oracle is pushing is Application Express which can produce very sophisticated web based applications. Another alternative is Java and ADF (application development framework) using JDeveloper.