You can select items from menus in different ways in Maya:
Almost all menu items are actions, but there are also many tools.
When you select an action menu item, that action is performed on the selected objects or components.
Some actions work differently depending on the order in which you select the objects. For example, you first select all the objects you want to constrain, and then select the constraining object last when you create a constraint.
If the order is important, instructions about the selection order appear in the help line at the bottom of the Maya window.
When you select a tool menu item, that tool is activated, which you can then use on objects or components. Instructions for using the tool appear on the help line when it's active.
The tool stays active until you exit it by selecting another tool.
Many items in the Curves and Surfaces menus can be converted from actions to tools (or vice versa) - see Switch operations between actions and tools.
For many menu items, you can set up options before executing it. When options are available, a box is displayed next to the name of a menu item. See Set or reset the options for a menu item for more information.
You can quickly repeat your last action from a menu item in Maya’s main menu, or Panel menu with the middle-mouse button. For example, if your last action was to open the Windows > Animation Editors > Graph Editor, then middle-click the Windows menu to reopen the Graph Editor. This also applies to sub-menus.