Proto-Sinaitic is a Middle Bronze Age script attested in a very small collection of inscriptions at Serabit el-Khadim in the Sinai Peninsula. Due to the extreme scarcity of Proto-Sinaitic signs, very little is known with certainty about the nature of the script. Because the script co-existed with Egyptian hieroglyphs, it is likely that it represented true writing, but this is by no means certain. It has also been argued that Proto-Sinaitic was an alphabet and the ancestor of the Phoenician alphabet, from which nearly all modern alphabets descend.
More information, and illustrations of the symbols, on the Wikipedia page