1. SDK Platform 可以理解为版本，因此有 SDK Platform 7，SDK Platform 8等等  Android SDK Tools 是各个版本都可通用的工具文件夹，里面有draw9patch  hierarchyviewer emulator等工具

Android SDK Platform-tools 是版本有区别的工具文件夹，里面有adb aapt等

这些文件夹都可以在sdk安装路径下找到

2. ARM EABI v7a System Image: 

更新Android 4.0 SDK后，创建4.0 AVD 时 ，报错：Unable to find a ‘userdata.img‘ file for ABI .svn to copy into the AVD folder 然后下载安装后没事了。因此可以推断，它应该是创建AVD(虚拟机)是管理用户信息的软件吧

if you work with the NativeDK, without that image the emulator is not able to simulate the execution of ARMv7 code (like multicore-instructions and NEON floating-point-unit)

3. Sources for Android SDK: 是这个版本的源代码。

4. Google APIS是google提供的一些api接口。

5. MIPS system image：

What is it?

It is an emulator image that emulates a device that has a MIPS CPU, instead of an ARM or x86 CPU.

What does it do and when will someone use it?

You will use it if you wish to emulate a device that has a MIPS CPU, instead of an ARM or x86 CPU.

What will happen if I do not install it?

You will not be able to emulate a device that has a MIPS CPU, instead of an ARM or x86 CPU. The MIPS Consortium may cry. But, considering right now there are very few Android devices with MIPS CPUs on the market, not having a MIPS emulator is probably just fine.

6. Intel x86 atom system image

So my question is, what is intel x86 atom system image in android sdk manager?

It is a version of the Android emulator that runs natively on x86 CPUs, like those in most development machines.

Contrast that with the ARM system image, which contains ARM (not x86) CPU instructions, and therefore must be translated when run.

Should i install it or not?

The x86 emulator images, where available, tend to run faster. You will have to do some work on your development machine to take advantage of them, as is outlined in the documentation.

the option is present in api level 15 & 16 but not in 17.

AFAIK Google relies upon Intel to create these images, and Intel has only done so on a few API levels.

Android SDK Manager下载详解