title: Angular2/4: What do [] and () means in angular html files?
body: <p>I'm having trouble understanding this. What does the following line mean?</p>
  
  <pre><code>&lt;virtual-scroll [items]="items" (update)="viewPortItems = $event"&gt;
  </code></pre>
  
  <p>1) What do the [] brackets means?</p>
  
  <p>2) Why is there a '=' after the [] brackets? What are we assigning to?</p>
  
  <p>3) What about the () brackets? Is it a function?</p>
  
  <p>4) MOST CONFUSING PART: Why is there an '=' sign next to "viewPortItems".. and then there is an '$event' sign which is an observable I presume. What's going on here?</p>