What is lightning:card in Salesforce?

Lightning:card tag (lightning component) is similar to apex:outputpanel (Visualforce page). It act as a container. 

A lightning:card is used to apply a stylized container around a grouping of information. The information could be a single item or a group of items such as a related list.

A lightning:card contains a title, body, and footer. To style the card body, use the Lightning Design System helper classes.

1. <aura:component >
2.     <lightning:card title="Container" iconName="standard:account">

3.      This is for Testing.

4.     </lightning:card>
5. </aura:component>

title attribute is required to use lightning:card.

iconName You can set icon name from this link.

Reference:

https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/aura_compref_lightning_card.htm