The actual process of passing data has already been answered, however most of the answers use hard coded strings for the key name in the Intent. This is usually fine when used only within your app. However, the documentation recommends using the EXTRA_* constants for standardized data types.

Example 1: Using Intent.EXTRA_* keys

First activity

Intent intent = new Intent(getActivity(), SecondActivity.class);
intent.putExtra(Intent.EXTRA_TEXT, "my text");
startActivity(intent);

Second activity:

Intent intent = getIntent();
String myText = intent.getExtras().getString(Intent.EXTRA_TEXT);

Example 2: Defining your own static final key

If one of the Intent.EXTRA_* Strings does not suit your needs, you can define your own at the beginning of the first activity.

static final String EXTRA_STUFF = "com.myPackageName.EXTRA_STUFF";

Including the package name is just a convention if you are only using the key in your own app. But it is a necessity to avoid naming conflicts if you are creating some sort of service that other apps can call with an Intent.

First activity:

Intent intent = new Intent(getActivity(), SecondActivity.class);
intent.putExtra(EXTRA_STUFF, "my text");
startActivity(intent);

Second activity:

Intent intent = getIntent();
String myText = intent.getExtras().getString(FirstActivity.EXTRA_STUFF);

Example 3: Using a String resource key

Although not mentioned in the documentation, this answer recommends using a String resource to avoid dependencies between activities.

strings.xml

 <string name="EXTRA_STUFF">com.myPackageName.MY_NAME</string>

First activity

Intent intent = new Intent(getActivity(), SecondActivity.class);
intent.putExtra(getString(R.string.EXTRA_STUFF), "my text");
startActivity(intent);

Second activity

Intent intent = getIntent();
String myText = intent.getExtras().getString(getString(R.string.EXTRA_STUFF));