A card shows one number in large type. It's the usual way to put headline figures, often called KPIs, at the top of a page.
When you drop a column into a visual, Power BI aggregates it. For numbers the default is Sum, which isn't always right: adding up unit prices gives a meaningless total, and you want the Average. Open the small arrow next to a field in a well to change it. The label changes with it, for example to "Average of Unit Price".
Text and dates get different choices. Text offers First, Last, Count and Count (Distinct). Dates offer Earliest and Latest. Count counts the rows that have a value; Count (Distinct) counts each different value once.