On a typical Linux system there will be at least two, if not more, directories or partitions meant to hold temporary files. There is always the /tmp directory, and often a /var/tmp directory as well. With newer Linux kernels, there can also be /dev/shm, which is mounted using the tmpfs filesystem.
One problem with directories meant to store temporary files is that they can often be targeted as places to store bots and rootkits that compromise the system. This is because in most cases, anyone (or any process) can write to these directories. Insecure permissions are problematic as well; most Linux distributions set the sticky bit on directories meant to contain temporary files — this means that user A cannot remove a file belonging to user B, and vice versa. Depending on the permissions of the file itself, user A may be able to view and/or modify the contents of that file, however.