When I log into my iCloud, my photo stream is not there. I have Photo Stream turned on, on my iPhone 4S.

1) Photo stream does not "show up" in icloud - there's no way to view the photos at icloud.com. Photo stream is just a sync service for getting copies of photos from device A to device B. It's not a gallery type of service.

2) When you turn on photo stream on a device, only subsequent photos are added to the stream (and appear on other devices that have the same stream account turned on).

3) Once in the stream, the servers keep the photos for up to 30 days, and only a max of 1000 photos. The service is not a storage or viewing service, it is for syncing.

And if the photo stream photos appear in icloud, they wont be erased ?

Think of Photo Stream as a "view" or "album" of your photos.

Photo Stream is visible on your devices, but not specifically on the cloud. the pictures are there, as they come from your regular photo stream.

The Stream is your last 1000 photos or 30 days worth, whichever happens first.

So again, the photos are on the cloud, but not singled out as your Photo Stream like on your devices. Like all synced photos, if you delete an image from your device, it will be removed from all albums and the Photo Stream.