What is the difference between spyware and adware?

Answer 1

Spyware is software that installs itself on your computer to spy on you

Adware produces all those unwanted popup ads. It is secretly installed when you download and install shareware or freeware.
Adware produces all those unwanted pop-up ads, and spyware is software that installs itself on your computer to spy on you and to collect personal information

Answer 2

Adware is considered a legitimate alternative offered to consumers who do not wish to pay for software. Programs, games or utilities can be designed and distributed as freeware.

Spyware is considered a malicious program and is similar to a Trojan Horse in which users unwittingly install the product when they install something else.

Spyware works like adware but is usually a separate program that is installed unknowingly when you install another freeware type program or application. Once installed, the spyware monitors user activity on the internet and transmits that information in the background to someone else. Spyware can also gather information about e-mail addresses and even passwords and credit card numbers.