• uname -a for all information regarding the kernel version,

  uname -r for the exact kernel version

• lsb_release -afor all information related to the Ubuntu version,

  lsb_release -r for the exact version

• sudo fdisk -l for partition information with all details.

  _{For more understandable information regarding paritions, please use commands given in other answers.}

Kernel Version

cat /proc/version             # detail about for the kernel image version

Distribution Version

lsb_release -a

Partition Sizes

cat /proc/partitions          # for basic sizes
sudo fdisk -l /dev/<device>   # eg /dev/sda

Commands:

• Kernel Version:

  uname -r
  

• Distribution version number:

  lsb_release -sr
  

• All partition sizes of the HDD in Terminal:

  lsblk -o NAME,SIZE
  

Example:

Kernel version is:

uname -r

Ubuntu release version is:

lsb_release -r

All partition size of the HDD in Terminal

df -ht ext4

replace ext4 with your FS type if it's not so

With respect to the marked as answer post:

Not correct uname -a return all not just kernel version also lsb_release -a return all not only Ubuntu version moreover sudo fdisk -l return the block numbers and size and not human readable information

Open terminal. Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

sudo fdisk -l; uname -a; lsb_release -a

uname -r

sudo fdisk -l

lsb_release -a

To find the exact distribution, that is Ubuntu 16.04.1 or Ubuntu 16.04.3 you would use

lsb_release -d