https://developer.salesforce.com/page/Asynchronous_Processing_in_Force_com Synchronous execution takes place as a single transaction and it does not wait for available resources. While asynchronous execution takes place depending on the resources available. @future, batch apex are examples of asynchronous execution. Governer limits in case of asynchronous are higher as compared to synchronous. In asynchronous operations; the code is executed in the background and the user is free to perform other operations.
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Synchronous execution takes place as a single transaction and it does not wait for available resources. While asynchronous execution takes place depending on the resources available. @future, batch apex are examples of asynchronous execution. Governer limits in case of asynchronous are higher as compared to synchronous. In asynchronous operations; the code is executed in the background and the user is free to perform other operations.