From BlenderWiki

Buttons and Controls

Buttons are mostly grouped in the Buttons window, but they can appear in other window types as well.

Operation Button

An operation button.

Operation buttons perform an operation when they are clicked (with LMB , as all buttons). They can be identified by their brownish color in the default Blender theme (see An operation button).

Toggle Button

Toggle buttons.

Toggle buttons come in various sizes and colors (see Toggle buttons). The green, violet, and gray colors do not change functionality, they just help the eye to group the buttons and recognize the contents of the interface more quickly. Clicking this type of button does not perform any operation, but only toggles a state.

Some buttons also have a third state that is identified by the text turning yellow (the Emit button in Toggle buttons). Usually the third state means “negative”, and the normal “on” state means “positive”.

Radio Buttons

Radio buttons are particular groups of mutually exclusive toggle buttons. No more than one radio button in a given group can be “on” at one time.

Number Buttons

Number buttons.

Number buttons (a.k.a. numeric fields) can be identified by their captions (see Number buttons), which contain a colon followed by a number. They are handled in several ways:

To increase the value, click LMB on the right of the button, where the small triangle/arrow is shown. To decrease it, click on the left of the button, where another triangle/arrow is shown.
To change the value in a wider range, hold down LMB and drag the mouse to the left (decreasing) or right (increasing). If you hold Ctrl while doing this, the value is changed in discrete steps (the step value depends on each button), if you hold ⇧ Shift, you’ll have finer control over the values, and if you hold both (Ctrl⇧ Shift), you’ll have ten times smaller discrete steps.
You can enter a value directly clicking ⇧ ShiftLMB (or hitting ↵ Enter while your mouse hovers the control). You can also enter simple equations, like 3*2 instead of 6. Be sure to enter a decimal point if you want a floating point result, otherwise you get an integer (e.g. 3/2 = 1, but 3.0/2 = 1.5). Handy geometric constants to remember: pi is 3.14 and the square root of two is 1.414. Press ⇧ Shift← Backspace to clear the value, Ctrl← to move the cursor to the beginning, and Ctrl→ to move the cursor to the end. Press Esc to restore the original value.
You can copy the value of a button by hovering it with the mouse and pressing CtrlC. Similarly you can paste the copied value with CtrlV.

Some number buttons contain a slider rather than just a number with side triangles. The same method of operation applies, except that single LMB clicks must be performed on the left or on the right of the slider, while clicking on the label or the number automatically enters keyboard input mode.

Menu Buttons

Use the menu buttons to choose from dynamically created lists. Menu buttons are principally used to link datablocks to each other (datablocks are structures like objects, meshes, materials, textures, and so on – see this page).

Datablock link buttons.

You can see an example for such a block of buttons in (Datablock link buttons).

The first button (with the tiny up and down pointing triangles) opens a menu (a.k.a. drop-down list) that lets you select the datablock to link to by clicking the desired item.
The second control, a text field, displays the type and name of the linked datablock and lets you edit this name after clicking LMB on it.
The “X” button clears the link.
The “auto” button generates an automatic name for the DataBlock.
The F button specifies whether the datablock should be saved in the file even if it is unused (unlinked), by creating a Fake user.

Unlinked objects

Unlinked data is not lost until you quit Blender. This is a powerful undo feature. If you delete an object, the material assigned to it becomes unlinked, but is still there! You just have to re-link it to another object or press its F button.

Color Selector controls

Color selector.

Some controls pop-up a dialog panel. For example, color controls, when clicked, will pop up a color selector dialog (see Color selector).

Cascade Buttons

Occasionally, some buttons actually reveal additional controls. For example, the Ramps panel has a cascade button called Colorband, that reveals additional controls dealing with colorbanding (see Colorband before and Colorband after).