Can someone please explain the difference between ViewState and Session?

More specifically, I'd like to know the best way to keep an object available (continuously setting members through postbacks) throughout the lifecycle of my page.

I currently use Sessions to do this, but I'm not sure if it's the best way.

For example:

SearchObject searchObject;
protected void Page_Load(object sender, EventArgs e)
{
     if(!IsPostBack)
     {
         searchObject = new SearchObject();
         Session["searchObject"] = searchObject;
     }
     else
     {
         searchObject = (SearchObject)Session["searchObject"];
     }
}

that allows me to use my searchObject anywhere else on my page but it's kind of cumbersome as I have to reset my session var if I change any properties etc.

I'm thinking there must be a better way to do this so that .NET doesn't re-instantiate the object each time the page loads, but also puts it in the global scope of the Page class?

Please advise. TIA