If not sure if you mean that if you delete songs from the library that they are removed from your computer or from your iPod.
If you mean the computer: Do you keep your songs in the iTunes Music folder or do you use another folder of your own making? iTunes will delete songs from the library and move them to Trash only if they are in the iTunes Music folder. (iTunes will ask if you want to trash the file if this is the case, it doesn't trash it automatically). If your songs are kept outside of this folder then deleting only removes the reference to the song from the library and you have to go to the folder they are located in to trash the file itself:
Removing items from iTunes -Windows
If you mean the iPod: If you want to delete songs from iTunes and keep them on the iPod you need to change the update option of your iPod to "manually manage music and videos" (this setting isn't available for an iPod shuffle, you can't delete from iTunes and keep the songs on the shuffle). The content of iTunes and the iPod are not syncronised in this mode so the two can be different, you can get details here:
Managing content manually on iPod and iPhone
iPod - Syncing Music
One major word of caution. If you delete songs or videos from your computer and have no back up other than the iPod you will risk losing them permanently if your iPod fails or you find you have to restore it to solve a problem. You should consider investing in an additional internal or external hard drive to store/back up your music. Alternatively at the very least back up anything you can't replace such as iTunes downloads to CD or DVD:
How to back up your media in iTunes
Something else to be aware of when using an iPod in manual mode is that the "Do Not Disconnect" message will remain on the display until you physically eject the device:
Safely Disconnect IPod
