While a watermark will not prevent people from sharing your images (the only guarantee for that is not to post them), it will identify the image as yours. A watermark is simple text, often partially transparent, across a photo. Most often, watermarking identifies the original source, copyright, or other use rights.
Most image editors make watermarking easy enough, but you can also skip that completely with a web service like PicMarker or an app, like iWatermark. If you have Photoshop, MakeUseOf has a great tutorial on watermarking.
If you host your images on Picasa, you can add a watermark as you upload. Flickr does not currently support watermarking but will allow you to upload watermarked photos -- provided they do not advertise (e.g. you may not use a domain name or URL).
Do you have a favourite service or technique for watermarking? Leave us a comment!