The key thing with a smartphone is what software it runs (like your computer needs Windows a phone needs software to keep it running).
The iPhone (from Apple) runs software called iOS.
Many other phones run Android software (owned by Google) and this is a huge growing market (Samsung sold far more Android phones last year than Apple sold iPhones).
Apple have their own app store, and Android have their own app store as well.
Blackberry does not run iOs or Android it runs it own software, but Blackberry is a shrinking market and far more people are likely to write apps for iOS and Android in the future than for Blackberry.
Apple iPhones are good but the charge a premium price.
Samsung Galaxy S2 (and the newer S3) are both excellent phones. The S3 runs the latest version of Android but will be more expensive than the S2.
If I were you I would jump ship from Blackberry and go for an Android.