<p>In Java, you often see a META-INF folder containing some meta files. What is the purpose of this folder and what can I put there?</p>

From
the official JAR File Specification
(link goes to the Java 7 version, but the text hasn't changed since at least v1.3):
The META-INF directory
The following files/directories in the META-INF directory are recognized and interpreted by the Java 2 Platform to configure applications, extensions, class loaders and services:
```
MANIFEST.MF
```
The manifest file that is used to define extension and package related data.
```
INDEX.LIST
```
This file is generated by the new "
```
-i
```
" option of the jar tool, which contains location information for packages defined in an application or extension.  It is part of the JarIndex implementation and used by class loaders to speed up their class loading process.
```
x.SF
```
The signature file for the JAR file. 'x' stands for the base file name.
```
x.DSA
```
The signature block file associated with the signature file with the same base file name. This file stores the digital signature of the corresponding signature file.
```
services/
```
This directory stores all the service provider configuration files.
New since Java 9 implementing
JEP 238
are multi-release JARs. One will see a sub folder
```
versions
```
. This is a feature which allows to package classes which are meant for different Java version in one jar.