A few ways to build clocks.

The ones listed here are all clocks I know so far. I inform myself times still further and post them then sometime to it 😉

• Command blocks (unusable in Survival)
• Lots of quartz and redstone (amplifier and comparator)

Instructions:

• Redstone comparator clock
  - Manufactures a Redstone comparator and places it on the ground.
  - Make a "ring" of redstone around it.
  - Sets the comparator with another right click to the mode subtract (visible in the F3 display).
  - Feed him an active redstone signal.
  My clock is with a lever, for turning on and off:
  
  [Note.] At activate of the lever, the clock activates!
                    At disable of the lever, the clock sends no Signal more off!
• Double hopper clock
  - Build two hoppers side by side so that the two "block feed tubes" always face the opposite hopper.
  - Places a Redstone comparator (and if desired, a Redstone amplifier) on one of the funnels.
  I've got my without Redstone amplifier built:
  
• Four funnel clock
  - Place four hoppers next to each other so that the "block feed tubes" point to the next hopper.
  - Puts on an Funnel a comparator.
  And if you want, an amplifier again:
  
• Redstone amplifier clock
  - Build four redstones on the ground in packs of 2, one block apart.
  - In between come the two different Directions Redstone amplifier.
  - Very, very briefly feeds an active redstone signal to the entity (e.g. set redstone flare and Sync and corrections by n17t01 dismantle again)
  This time I can't say anything:
  
• Redstone Amplifier Clock Vol. 2
  - Build redstone in the corners of a 3x3 block hex.
  - Sets the amplifiers so that the "energy flow" is in one direction.
  - Anyone Amplifier must be set to at least 1 tick delay, otherwise the signal will "roll over" and "energize" everything:
  
  
• Command block clock
  [VERALTET] Since the 1.9 there are repeat command blocks! These partially replace this clock ^{(partially, because the command block clock is executed every tick, depending on the load the execution speed of the 1.9 command blocks changes, the clock here stays permanently this fast)}!
  - To obtain the command blocks, type /give [your MC name] command_block as a command in chat.
  - Set a command block on the bottom and enter /setblock ~ ~1 ~ air with a right click. 2 blocks above another command block is set, in which /setblock ~ ~-1 ~ redstone_block is entered, otherwise it does not work; also not the other way around, so the command of the lower block into the upper or even lying (on the ground, sideways) it does not work.

• Slime Piston Clock
  - Sets an upward sticky Flask on the ground and place a block of slime on it.
  - Place a redstone block on the left side of the slime block.
  - Ensures that the piston gets a block update (see picture below: lever)
  
  [Note.] At activated The lever is and remains the piston extended!
  At deactivated Lever is the clock activates and the piston goes up and down. I do not know why, but OK XD!
  Image:
  
  [Note.] I got the idea to upload this summary from this Page because she inspired me to put one up just like it! She is from SirTurtle