A pad is generally a surface, but there are many different kinds of pads:
You can take notes on a notepad or on a pad of paper.
notepad / pad of paper
Many people use a mouse pad under a mouse.
A frog rests on a lily pad.
A mattress pad provides additional comfort to a mattress.
Anything that is padded is usually more comfortable.
She’s wearing knee and elbow pads for protection.
knee and elbow pads
He’s moving into his new pad.
(Sometimes the word "pad" is used in place of the noun, "apartment.")
a new pad
You can also used the word "pad" as a verb. In this case, it means to add more:
She’s padding her salary by doing extra work around the office and collecting overtime.
They’re going to pad their chairs with new cushions.
The bird padded her nest with dry grass.
The word "padded" is an adjective:
A padded chair is more comfortable than one that it unpadded.
a padded chair
Note: An iPad is a tablet computer made by Apple. The reason why they named it "iPad" is because it can function as a notepad does, but there are many things that you can do with an iPad other than take notes.