Send SMS buttons work on record detail pages in Salesforce and allow you to send SMS by clicking on the button (Lead or Contact if it’s a Person Object or associated record like Cases, as per the MoC Configuration). It sends SMS to the Mobile field associated with the button and also gives the capability to use a template with merge fields from associated objects.
Send Bulk SMS on list view layout gives you the capability to select multiple records and send SMS to those records. You can compose a new SMS or choose a single template with merge fields from associated objects to send to all the recipients.
You can create Send SMS and Bulk SMS buttons and associate these buttons to Visualforce pages generated in the configuration of message objects. For more information about setting up SMS buttons on page layout, search layout, and so on, see Configure SMS Button.