The group option folds the rows of any data tag into one occurrence per distinct value of a field. Each group holds that value, $count - the number of rows in the group - and rows, the group's own rows as nested data. Inside {rows} a field is the row's own value again.

{data 'orders'}
  [ {"customer":"Ann","number":101,"total":40}, {"customer":"Bob","number":102,"total":15},
    {"customer":"Ann","number":103,"total":25}, {"customer":"Cleo","number":104,"total":60},
    {"customer":"Bob","number":105,"total":35} ]
{/data}

{orders group='customer', sum='total'}
  <h4>{$customer}: {$count} orders, {$total}</h4>
  {rows}
    <p>Order {$number}: {$total}</p>
  {/rows}
{/orders}

Aggregates

sum, avg, min and max name the fields to aggregate over each group's rows. The aggregate is stored under the field's own name - {$total} above is the sum - and under the aggregate joined to it - {$sum_total}, {$avg_total} - so one field can be summed and averaged at once. A field list such as sum='total, quantity' aggregates each of them.

{data 'orders'}
  [ {"customer":"Ann","number":101,"total":40}, {"customer":"Bob","number":102,"total":15},
    {"customer":"Ann","number":103,"total":25}, {"customer":"Cleo","number":104,"total":60},
    {"customer":"Bob","number":105,"total":35} ]
{/data}

<table>
  <tr><th>Customer</th><th>Orders</th><th>Total</th><th>Average</th><th>Smallest</th><th>Largest</th></tr>
  {orders group='customer', sum='total', avg='total', min='total', max='total', sort='total DESC'}
    <tr>
      <td>{$customer}</td><td>{$count}</td><td>{$total}</td>
      <td>{$avg_total}</td><td>{$min_total}</td><td>{$max_total}</td>
    </tr>
  {/orders}
</table>

In order with the other options

Handling options run in the order they are written. Groups come in the order their first row appears: a sort written before the group orders the groups and the rows inside them, a sort written after it orders the groups - also on an aggregate, as above. first, page and where written after the group count groups: {orders group='customer', where='$count gt 1'} keeps the customers with more than one order. Several fields - group='year, month' - group on the combination, and {rows group='month'} groups again inside a group.