Be brief but be sure you give the audience the proper orientation for the body of the talk. Trends Continuous line graphs show trends or correlations effectively. Be sure to label both X and Y axes. Make sure the graph has enough information to be understood without a lengthy explanation of its details - simply interpret it. Comparisons and proportions Bar graphs are best for comparing magnitudes.
Pie charts are good for showing relative parts of the whole.
Symbols Symbolic diagrams of circuits or flow charts are useful if carefully prepared and if not too detailed. Use IEEE standard symbols where applicable. Otherwise, define your symbols. Flow and relationship Simple flow charts or schematic diagrams can convey flow or relationships to be described. Show only those parts or details necessary to explain how a thing works.