ToolStripSplitButton vs. ToolStripDropDownButton

Split buttons are regular buttons with a drop-down menu attached to them. If you click a split button on the button part, it acts like a regular button. If you click it on the menu part, a menu drops down. The "back" button on the majority of browsers are this type of button, for example.

Drop-down buttons, when clicked, drop down a menu. They don't have (and shouldn't have) directly associated functionality beyond displaying the drop-down menu. They are like the topmost items in a menu: they don't do anything when clicked other than to show you more items.

quoted from here

Think of a ToolStripSplitButton as a regular button joined to a ToolStripDropDownButton.

When you click on a ToolStripDropDownButton the Click event always fires and the drop down list is displayed.

When you click on the button side of the ToolStripSplitButton the Click event fires but the drop down list is not displayed and if you click on the drop down side of the split button the drop down list is displayed but the Click event does not fire.

In Internet Explorer 6 the Mail button acts like a ToolStripDropDown button whereas the Edit button acts like a ToolStripSplitButton - i.e., if you click on the image it just fires up the editor wheras if you click on the arrow it offers a drop down list of editors.

Visually, the cue when you hover the cursor over the button is that a ToolStripSplitButton will show a solid vertical line dividing the image from the drop down arrow (the two sides of the 'split') whereas the ToolStripDropDown button will not show a vertical line.