System Monitors for Linux.

(Note in Xubuntu or Ubuntu 18.04.x you can drop the "get" on sudo apt-get.)

The built-in system monitor in Linux is pretty good much better than Windows built-in.

If you have a menu - Settings - "System Monitor" Use the "Resources" Tab.
No menu "Unity garbage" search for "System Monitor"
The built-in System Monitor is pretty good.
CPU use all Cores/CPU's (Threads), Memory/Swap use (If you use a swap file), Internet speed, down rate total downloaded, upload rate total uploaded.

GKrellM
I personally like this one "GKrellM System Monitor". If you still have the "Software Center" you can find it there.
By default Ubuntu 18.04 does not have the Software Center it's been replaced with stuff that just doesn't work that well.
1. Open a Terminal Window
2. Type in or copy & paste into the terminal window and hit enter: sudo apt-get install gkrellm
3. Menu - Settings, no menu search for GKrellM


htop runs in terminal
1. Open a Terminal Window
2. Type in or copy & paste into the terminal windows and hit enter: sudo apt-get install htop
3. To start type in or copy & paste into the terminal window and hit enter: htop


Monitorix - web based runs in the browser.
1. Open a Terminal Window
2. Type in or copy & paste into the terminal window and hit enter: sudo add-apt-repository ppa:upubuntu-com/ppa
3. Type in or copy & paste into the terminal window and hit enter: sudo apt-get update
4. Type in or copy & paste into the terminal window and hit enter: sudo apt-get install monitorix
5. To start Type in or copy & paste into browser address bar hit enter: http://localhost/monitorix/