dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile project(':social_sdk_library_project')
    compile 'com.android.support:appcompat-v7:22.2.0'
    compile 'com.github.chrisbanes.photoview:library:1.2.3'
    compile 'com.android.support:support-v4:22.2.0'
    compile 'com.jakewharton:butterknife:6.1.0'
    compile 'com.networkbench.newlens.agent.android:nbs.newlens.agent:2.2.5'
    compile 'com.google.zxing:core:3.2.0'
    compile 'com.joanzapata.android:android-iconify:1.0.9'
    compile 'com.loopj.android:android-async-http:1.4.8'
}

如上面的依赖中的包，比如com.loopj.android:android-async-http:1.4.8，存放在哪儿的？

compile fileTree 和 compile project我知道什么意思，而直接的compile是什么意思？