There is also a GuiMain object but it is deprecated now. They are put into StarterGui or are created by a script.
BillboardGui
The BillboardGui is a special GUI object, it is attached to a BasePart so it is always above/under/next to a part. To attach a BillboardGui to a Part instance, set The BillboardGui's Adornee property to the part.
Frame
The Frame represents a rectangle on the screen. Most other GUI objects are based on the Frame.
TextLabel
The TextLabel represents a Frame with a label. To create a TextLabel that just displays the text, set it's Size to 0 x 0 pixels or it's BackgroundTransparency to 1.
TextButton
The TextButton looks like a TextLabel, but retrieves 6 additional events (MouseButton1Click, MouseButton1Down, MouseButton1Up, MouseButton2Click, MouseButton2Down, MouseButton2Up) which are fired when the player clicks on the TextButton.
TextBox
In addition to the events and properties of TextLabel, the text of a TextBox can be changed by the Player online. To do so, the player has to click on the TextBox, write something and then press enter. The TextBox only supports lower case characters, numbers and some other character like ",", ".", "/" etc.
ImageLabel
Is a Frame, that shows an image that is placed over the background of the ImageLabel and streched to it's Size.
ImageButton
The ImageButton is just an ImageLabel with the same events as TextButton.
Like all objects in Roblox, you can create an instance of those GUI objects with Instance.new(className).
Coordinates
The coordinates of the Roblox GUI have 4 values: X.Scale, X.Offset, Y.Scale and Y.Offset. To create a new coordinate value use UDim2.new, this function takes 4 arguments and returns a userdata.
The point UDim2.new(0,0,0,0) is the upper left corner of your screen (0|0), the highest x coordinate is at the right side of your screen and the highest y coordinate is at the bottom. Something would be at 10|10.
Calculating the absolute position or size.
Even if there is are the properties AbsolutePosition and AbsoluteSize which contain those values, you should know how to
calculate them, so you can understand Roblox' system for sizes and positions better.
Let's start with two frames. The first one will be in a ScreenGui object, the second will be inside the first. Then we make the second one red it should be the same colour.
The AbsoluteSize of a ScreenGui is equal to the size of your window or rather the part used for the 3D world, it's AbsolutePosition is 0|0.
Now let's check it.
absolute_size_x = 0 + (0.5 * 200) + 0 -- the position was not set, it is 0|0 by default.
absolute_size_y = 50 + (0.5 * 100) + 0
print(absolute_size_x,",",absolute_size_y)
You should get this output:
100 , 100
The size of frame1 was 200,100 so frame2 should fit in one helf of it.
Colors
Instead of RGBA values, GUIs use RGB values and a special property called BackgroundTransparency (or for text TextTransparency). The properties holding the RGB (in Roblox Color3 values) are BackgroundColor3, BorderColor3 and TextColor3. Those name should be self-explanatory. If you want to use BrickColor values use BackgroundColor, BorderColor and TextColor instead. BackgroundTransparency set the transparency for the background and the border, TextTransparency set the transparency for the text. 1 is completely transparent, 0 is non-transparent, 0.5 is 50% transparent etc.
To create a color value use Color3.new or BrickColor.new
local my_color = Color3.new(r, g, b)
local my_brick_color = BrickColor.new(color)
frame.BackgroundColor = BrickColor.new("Bright red")
frame.Size = UDim2.new(0, 100, 0, 100)
frame.Position = UDim2.new(0.5, -50, 0.5, -50)
frame.Parent = script.Parent
while true do
for i=1,10 do
frame.BackgroundTransparency = i/10
wait(.1)
end
wait(1)
for i=0.9,0,-0.1 do
frame.BackgroundTransparency = i
wait(.1)
end
wait(1)
end
Popup
button = Instance.new("TextButton")
Example3
button.Size = UDim2.new(0, 100, 0, 25)
button.Text = "Click me!"
button.Parent = script.Parent
button2 = Instance.new("TextButton")
button2.Size = UDim2.new(0, 50, 0, 50)
button2.Position = UDim2.new(0.5, -25, 0.5, -25)
button2.Text = "Close"
button2.Visible = false
button2.Parent = script.Parent
function button_Click()
button2.Visible = true
end
function button2_Click()
button2.Visible = false
end
button.MouseButton1Click:connect(button_Click)
button2.MouseButton1Click:connect(button2_Click)
-- click on both buttons!
square.Parent = script.Parent
square.Size = UDim2.new(0, 20, 0, 20)
while true do
for i=0,50 do
square.Position = UDim2.new(0, i * 5, 0, i * 5)
wait()
end
for i=50,0,-1 do
square.Position = UDim2.new(0, i * 5, 0, i * 5)
wait()
end
end
ScreenGui contains all GUI objects a player can see. They must be placed inside a 
Player's 
