Logメソットの使い方

Logメソットの使い方の使い方がわかりません。

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageView;
import android.widget.TextView;
import android.support.v4.app.NavUtils;

public class MainActivity extends Activity {

    private final String TAG = "Chapter0514";

    private int mCount = 0;

    private ImageView mImageView1;

    private TextView mTextView1;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // âÊñ ÇÃÉCÉìÉXÉ^ÉìÉXÇéÊìæ
        mImageView1 = (ImageView) findViewById(R.id.imageView1);
        mTextView1 = (TextView) findViewById(R.id.textView1);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
    protected void onDestroy(){
        super.onDestroy();
        Log.d(TAG,"onDestroy count="+mCount);
    }

    protected void onRestoreInstanceState(Bundle savedInstance){
        super.onRestoreInstanceState(savedInstance);
        Log.d(TAG,"nRestoreInstanceState count="+mCount);
    }

    protected void onSaveInstanceState(Bundle outState){
        super.onSaveInstanceState(outState);
        outState.putInt("count",mCount);
        Log.d(TAG,"nRestoreInstanceState count="+mCount);
    }

    protected void onPause(){
        super.onPause();
        Log.d(TAG,"onPause count="+mCount);
    }

    @Override
    protected void onResume(){
        super. onResume();
        Log.d(TAG,"onPause count="+mCount);
    }

    protected void onStart(){
        super. onStart();
        Log.d(TAG,"onPause count="+mCount);
    }

    protected void onStop(){
        super. onStop();
        Log.d(TAG,"onPause count="+mCount);
    }


}

のようにコードを書いたのですが、例えば以下のようなLogメソットのところの構成がわかりません。

protected void onDestroy(){
        super.onDestroy();
        Log.d(TAG,"onDestroy count="+mCount);
    }

LogメソットのdとTAGが何を表しているか、わかりません。