Once you upload the photos, you can categorize them, arrange them into sets and give them descriptions. The manner of organizing the photos depends on the site. Some use tags, which are keywords that you can assign to each photo. When someone wants to look at your photos, they can see a list of your tags and choose to look at all your photos with certain keywords. If they only wanted to see photos of horses that you took while on vacation, they could combine the tags "horse" and "vacation." Often, the list of tags will show which categories hold the most photos by showing that tag in a larger font size.
Alternately, you can arrange photos into sets. You can set the specific order of the photos, so visitors to your sharing site will see them like a slideshow (some sites have an actual slideshow viewing option).
The descriptions on each photo can include as much information as you like. The camera used, the location of the photo, the subject of the photo, or any interesting stories about the photograph are all possible in the description.
Most digital cameras encode information about the photo and the camera itself into the image files. This is EXIF data (Exchangeable Image File Format), and when you upload your photos to a site, this information can be included automatically. This will allow users to see what camera you used, the time and date the photo was taken and the camera settings used for the photo, all without you having to enter any of the data yourself. Of course, you can disable this if you don't want people to know it.
When you upload your photos to a photo-sharing site, you aren't necessarily putting them out there for the entire world to see (although you can). It is possible to set permissions on each photo. Anyone can view photos set to "public." Set a photo to "private" and it will only display for people who you allow to see it. On some sites, the "private" category covers everyone on your contacts or friends list. You can also create different categories of people, and only allow certain categories of them to see certain photos. You might want anyone you know to see your nature pictures, but you don't want your co-workers see the photos you took at your family reunion. So you can put the co-workers in their own category and exclude them from those pictures.
Finally, you can disable downloading on some or all of your photos. This prevents people from saving the photo to their own computer. However, technically speaking, if an image is displayed on someone's computer, they can find a way to download it. Flickr puts a transparent image over your photo to hinder downloading, but it can't stop people completely from downloading.