参考：AndroidSDK/docs/reference/android/content/Intent
看到相关代码，对这两个参数的区别有点搞不清。
原文地址请保留http://www.cnblogs.com/rossoneri/p/3985597.html
现在知道的是new Intent(Intent.ACTION_GET_CONTENT, null)必须要setType("type/后缀")，否则会会弹出异常FATAL EXCEPTION:main android.content.ActivityNotFoundException，
ACTION_PICK可以不用setType()。但建议设置一下属性，因为他们都是通过属性来打开相应的应用程序。
专门从文档摘下来说明，放一起比较下，以后再看这方面的说明也方便。就不翻译了。程序猿嘛，总得看的懂英文文档吧。
public static final String ACTION_GET_CONTENT
Activity Action: Allow the user to select a particular kind of data and return it. This is different than ACTION_PICK in that here we just say what kind of data is desired, not a URI of existing data from which the user can pick. A ACTION_GET_CONTENT could allow the user to create the data as it runs (for example taking a picture or recording a sound), let them browse over the web and download the desired data, etc.
There are two main ways to use this action: if you want a specific kind of data, such as a person contact, you set the MIME type to the kind of data you want and launch it with startActivity(Intent). The system will then launch the best application to select that kind of data for you.
You may also be interested in any of a set of types of content the user can pick. For example, an e-mail application that wants to allow the user to add an attachment to an e-mail message can use this action to bring up a list of all of the types of content the user can attach.
In this case, you should wrap the GET_CONTENT intent with a chooser (through createChooser(Intent, CharSequence)), which will give the proper interface for the user to pick how to send your data and allow you to specify a prompt indicating what they are doing. You will usually specify a broad MIME type (such as image/* or */*), resulting in a broad range of content types the user can select from.
When using such a broad GET_CONTENT action, it is often desirable to only pick from data that can be represented as a stream. This is accomplished by requiring the CATEGORY_OPENABLE in the Intent.
Callers can optionally specify EXTRA_LOCAL_ONLY to request that the launched content chooser only returns results representing data that is locally available on the device. For example, if this extra is set to true then an image picker should not show any pictures that are available from a remote server but not already on the local device (thus requiring they be downloaded when opened).
Input: getType() is the desired MIME type to retrieve. Note that no URI is supplied in the intent, as there are no constraints on where the returned data originally comes from. You may also include the CATEGORY_OPENABLE if you can only accept data that can be opened as a stream. You may use EXTRA_LOCAL_ONLY to limit content selection to local data.
Output: The URI of the item that was picked. This must be a content: URI so that any receiver can access it.
public static final String ACTION_PICK
Activity Action: Pick an item from the data, returning what was selected.
Input: getData() is URI containing a directory of data (vnd.android.cursor.dir/*) from which to pick an item.
Output: The URI of the item that was picked.