An app icon is a very important to get a first impression for your audience to make them download an app. Most people loose interest in downloading the app if the Icon looks dull. If you have a million downloads for your app no body cares for your App Icon. But if you are a new kid in the block in the App market , you would need a great icon to leave a great impression.
So, what do you need to make a great app icon? You can use either Adobe Photoshop or Adobe Illustrator to make the app icons.
Photoshop gives you a raster image ( jpeg/png image) while Adobe Illustrator gives a Vector Image ( svg/eps/ai file) that can be scaled to any size without loosing the quality.
With multiple sized devices coming into market every day , you need to have your app icon in multiple sizes like 28*28 , 56*56, 128*128, 256*256 etc. If you have used a Photoshop to make the app icon , then it would be very tough to maintain the same quality for all different sized images.
But if you use a Adobe Illustrator and create a vector image for your app icon , then the same vector file can generate different sizes of same image without loosing any quality.
So, start using vector images for making your app icons. Vector Images can be designed using multiple tools like Adobe Illustrator or Coreldraw.