Session in Oracle is in simple is when you logon into Oracle everytime it is a session.

Batch as it can be defined as a group of something.   

Session:  A session is a set of commands that describes the server to move data to the target.

Batch :  A Batch is set of tasks that may include one or more numbar of tasks (sessions, ewent wait, email, command, etc..,)

There are two types of batches in Informatica:

1. Sequential: When Data moves one after another from source to target it is sequential

2. Concurrent: When whole data moves simultaneously from source to target it is Concurrent

A session is a set of instructions that tells the Integration Service how and when to move data from sources to targets. A session is a type of task, similar to other tasks available in the Workflow Manager. In the Workflow Manager, you configure a session by creating a Session task. You can run as many sessions in a workflow as you need. You can run the Session tasks sequentially or concurrently, depending on the requirement.

When you create a Session task, you enter general information such as the session name, session schedule, and the Integration Service to run the session. You can also select options to run pre-session shell commands, send On-Success or On-Failure email, and use FTP to transfer source and target files.

Batch :
Batch is a group of sessions, There are three types of batch
Sequential batch
Parallel batch
Control flow