barrier is a block that has a red sign on it.it blocks you from passing a area where your not allowed to get passed.
if you put barrier and redstone on it 3x3 (3 wide 3 tall) on all barrier blocks it will dissapier.and if you remove them they wont appear again.if you place one close to it/or farther it will appear again.it is used for mazes and other stuff e.g parkours.
the give command is /give [name] minecraft:barrier and hit enter and you'll get a barrier.you can use it as a invisible wall
e.g placing signs on it,and placing ladders on it.

slimeblock is a crafted block in snapshot 1.8. you can use it for elevators.it is crafted by putting 9 slime balls into all slots in a 
crafting table.in the early snapshots  14w31a or earlier snapshots theres a glitch with the slimeblocks sometimes it pushes you out and i didn't find a problem with that. command:/give [name] minecraft:Slime_Block

download the launcher :https://mojang.com/2013/07/minecraft-the-horse-update/ scroll down till you see get launcher here
download it.enjoy