Shortcuts keys let you perform task in a much less time then performing that using a mouse. You can even add you own custom shortcuts in Ubuntu but lets see the default shortcuts.

So here is a list of keyboard shortcut keys in Ubuntu which you can use to perform different tasks.

Ctrl + A : Select all
Ctrl + C : Copy the highlighted content to clipboard
Ctrl + V : Paste the clipboard content
Ctrl + N : New (Create a new document, not in terminal)
Ctrl + O : Open a document
Ctrl + S : Save the current document
Ctrl + P : Print the current document
Ctrl + W : Close the close document
Ctrl + Q : Quit the current application

Shortcuts for Terminal

Ctrl + A : Move cursor to beginning of line
Ctrl + E : Move cursor to end of line
Ctrl + C : kills the current process.
Ctrl + Z : sends the current process to the background.
Ctrl + D : logs you out.
Ctrl + R : finds the last command matching the entered letters.
Enter a letter, followed by Tab + Tab : lists the available commands beginning with those letters.
Ctrl + U : deletes the current line.
Ctrl + K : deletes the command from the cursor right.
Ctrl + W : deletes the word before the cursor.
Ctrl + L : clears the terminal output
Shift + Ctrl + C : copy the highlighted command to the clipboard.
Shift + Ctrl + V (or Shift + Insert) : pastes the contents of the clipboard.
Alt + F : moves forward one word.
Alt + B : moves backward one word.
Arrow Up/Down : browse command history
Shift + PageUp / PageDown : Scroll terminal output

For Firefox:

CTRL+L : Firefox Address Bar
CTRL+pgup : Next Tab (left to right)
CTRL+pgdn : Previous Tab (right to left)
CTRL+T : New Tab
CTRL+R / f5: Reload Page
CTRL+U : View Page Source

Shortcuts for Nautilus

Shift + Ctrl + N : Create New Folder
Ctrl + T : Delete selected file(s) to trash
Alt + ENTER : Show File/Folder Properties
Ctrl + 1 : Toggle View As Icons
Ctrl + 2 : Toggle View As List
Shift + Right : Open Directory (Only in List View)
Shift + Left : Close Directory (Only in List View)
Ctrl + S : Select Pattern
F2 : Rename File
Ctrl + A : Select all files and folders
Ctrl + W : Close Window
Ctrl + Shift + W : Close All Nautilus Windows
Ctrl + R : Reload Nautilus Window
Alt + Up : Open parent directory
Alt + Left : Back
Alt + Right : Forward
Alt + Home : go to Home folder
Ctrl + L : go to location bar
F9 : Show sidepane
Ctrl + H : Show Hidden Files
Ctrl + + : Zoom In
Ctrl + - : Zoom Out
Ctrl + 0 : Normal Size

For Gnome

Ctrl + Alt + F1 : Switch to the first virtual terminal
Ctrl + Alt + F2(F3)(F4)(F5)(F6) : Select the different virtual terminals
Ctrl + Alt + F7 : Restore back to the current terminal session with X
Ctrl + Alt + Backspace : Restart GNOME
Alt + Tab : Switch between open programs
Ctrl + Alt + L : Lock the screen.
Alt + F1 : opens the Applications menu
Alt + F2 : opens the Run Application dialog box.
Alt + F3 : opens the Deskbar Applet
Alt + F4 : closes the current window.
Alt + F5 : unmaximizes the current window.
Alt + F7 : move the current window
Alt + F8 : resizes the current window.
Alt + F9 : minimizes the current window.
Alt + F10 : maximizes the current window.
Alt + Space : opens the window menu.
Ctrl + Alt + + : Switch to next X resolution
Ctrl + Alt + - : Switch to previous X resolution
Ctrl + Alt + Left/Right : move to the next/previous workspace

These are the list of shortcuts i know for ubuntu and if you know more of them, share with us