I installed Tortoise SVN and configured it with Aptana Studio. When I right-click on the folder that I have added into Tortoise repository, it shows the Check-Out option, but not the Check-In option. Why?
Even in Aptana IDE, it is not showing Check-In.
Either, as hexium says, you're looking for the wrong thing, or you have it hidden in a submenu. Explore the settings option inside of the SVN submenu, to choose whether commit is displayed on the root context menu or in a submenu.
In other version control systems, you have to get a local working copy of the repository, which is initially read-only. Then you have to explicitly "check out" a file before you can edit it and then you "check in" once you're done.
In SVN terms, however, "check out" is the operation of creating a local working copy of the repository (or a subtree of it). Once you have the working copy, it is already editable; you don't need to do an explicit action before you can edit the file. SVN will track automatically whether the file was modified locally and once that happens, SVN will offer you "commit" option, which will submit your changes to the repository.