Yes. They will be offline index rebuilds, meaning that changes to the index/table could be blocked during the operation.

As noted above blocking will occur during the index rebuild of each index/table. Overall performance may be seen since these involve I/O operations. You will generally do these during your maintenance windows for the server or application.

While maintenance plans are the easier way to implement index maintenance, and are better than nothing, it would be advised to look at using customized scripts. The maintenance plan offers little customization as to which indexes actually need to be rebuilt or not. Ola Hallengren offers the most used scripts for index and statistics maintenance of a database.