if the partition was created using boot camp assistant its best to remove it using bot camp assistant itself ( as mentioned by woodyz ). Regardless how the partition was created first try with boot camp assistant. If Boot camp assistant says there are no partitions use disk utility as i mentioned earlier.
to use boot camp assistant :
Go to applications folder in finder > utilities > boot camp assistant.
you will have to check or uncheck relevant options to remove the partition.
Merging will be done automatically by the assistant wizard.