What are the Batch apex governor limits and Best practices?

Batch Apex Best Practices and Limits

• All methods in the class must be defined as global.
• Methods declared as future aren't allowed in classes that implement the Database.Batchableinterface.
• Methods declared as future can't be called from a batch Apex class.
• You cannot call the Database.executeBatch method from within any batch Apex method.
• Up to five queued or active batch jobs are allowed for Apex. 
• A user can have up to five query cursors open at a time.
• When you call Database.executeBatch, Salesforce only places the job in the queue at the scheduled time. Actual execution may be delayed based on service availability.
• A maximum of 50 million records can be returned in the Database.QueryLocator object. If more than 50 million records are returned, the batch job is immediately terminated and marked as Failed.
• When testing your batch Apex, you can test only one execution of the execute method. You can use the scope parameter of the executeBatch method to limit the number of records passed into the execute method to ensure that you aren't running into governor limits.
• The maximum value for the optional scope parameter is 400. If set to a higher value, Salesforce chunks the records returned by the QueryLocator into smaller batches of 400.
• If no size is specified with the optional scope parameter, Salesforce chunks the records returned by the QueryLocator into batches of 200, and then passes each batch to the execute method. Apexgovernor limits are reset for each execution of execute.
• The start, execute and finish methods can implement only one callout in each method.
• Batch executions are limited to one callout per execution.
• The maximum number of batch executions is 250,000 per 24 hours.
• Only one batch Apex job's start method can run at a time in an organization. Batch jobs that haven’t started yet remain in the queue until they're started. Note that this limit doesn’t cause any batch job to fail and execute methods of batch Apex jobs still run in parallel if more than one job is running.
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