Liter is a volume measurement unit and commonly used globally except a few countries. Litre (liter is American spelling) was the base volume unit in the old metric system and it was defined as the volume of one kilogram of water. As this often causes confusion, please note that liter is not an SI (International System of Units) unit but since commonly used worldwide, accepted for use within SI. It equals to 1 cubic decimeter. It's commonly used in everyday life to measure the fluids. The abbreviation is "l".
Fluid ounce is a volume unit in imperial and U.S. customary measurement systems. Ounce (as a word) is also used as a mass (weight) unit but when used as a volume unit, fluid ounces are meant. After the metric system is adopted in many countries, it's usage is replaced with grams and milligrams but still one of the main mass (weight) units in United States and Britain and a few other countries. The abbreviation is "fl oz".