Approval - Group and Approval - User are both workflow activities that generate approval requests, but in different ways:
The Approval - User activity will create one or more approval requests for individual users. You can add both users and groups as approvers in this activity, but when adding a group, this activity will actually generate a separate approval request for each user in that group:
If you were to check the Advanced box, you could even use a script to generate additional users for approval, by setting a pre-defined variable (answer) to an array of Sys IDs corresponding to the list of users you'd like to request approval from (in addition to those specified in the Users and Groups fields.
In general, it's always best to either specify a group, or use a script to programmatically determine who the approvers should be, rather than hard-coding a list of users. Imagine if the user we've hard-coded as the approver for a certain type of request, leaves the company abruptly. Most companies have a development and approval cycle that takes days, if not weeks, to deploy a change to production, including workflow changes. However, certain things in ServiceNow are considered data, rather than configuration; data can generally be updated directly in production, whereas configuration (including workflows) generally cannot.
While workflows cannot usually be updated without a change request and approval, group membership and ownership can; so if we can use a script to grab the owner of a certain group, then all we have to do is make sure that if one person leaves the company, any groups that they manage have someone else put in the management field instead, so that approvals continue to function.
User approvals, like group approvals, also allow you to set the conditions under which the approval request is considered approved (or rejected). You can set it to Wait for: Anyone to approve, everyone to approve, the first response from any individual, of a condition based on a script. In a script, you could determine the total number of approvers, and then wait for more than 50% of them to approve or reject. Alternatively, you can set it so that if anyone rejects, then the entire approval request is rejected.
Finally, you can even set a Due date, so that users must reply within a specified duration.
The Approval - Group activity is highly similar to Approval - User, except that you can only specify groups (not individual users) to approve, and the Wait for conditions are more group-oriented: An approval from each group, an approval from any group, approval from everyone in all groups, and so on:
