A simple lag in your computer is noticeable and annoying. When your RAM is hosting inactive data (for eg: Openoffice, unused tabs from browser, media player paused etc) it tends to move it to swap partition which is in most cases a hard drive which is much slower than your RAM. The free’d RAM is used for active processes.
This all seems smart & obvious, but if you check your RAM usage by

free -m

you will notice that almost half of the RAM is available, and if you observe carefully you will find that your swap is used after some percentage of RAM is used. To control the swap usage swappiness value is used. To check extent of swap usage for your machine just issue

cat /proc/sys/vm/swappiness

This will print 60 for default ubuntu installation. The value can be anything between 0 to 100. 0 means no
swap usage and 100 means use extensively. A value of 20 will works well for machine with 2 GB or more RAM. To set value permanently, open the following file

gksudo gedit /etc/sysctl.conf

and enter this value at the end of the page

vm.swappiness=10

save the file and reboot your machine.