The left side-bar of Scheme-it contains a useful library of schematic symbols and diagram symbols for use in your drawings. It has five different sections: Reference Designs, Schematic Symbols, System Blocks, System Icons, Diagram Symbols, Custom Symbols, Manufacturers and Import BOM.
To place one of these symbols, click on a specific category to open a specific symbol list. Then, click or click-and-drag on the symbol itself (using your left mouse button) to place it in your diagram. Press and hold the shift key to place multiple components on the schematic area.