what does something::something means in yii?

I'm a newbie in programming and now I'm learning yii framework and it's really hard.

What does this declaration mean :
something::something

for instance : CHtml::encode, yii::appname, CHtml::dropDownList, etc...

    Yii::app()->request->baseUrl;

    CHtml::encode($data->getAttributeLabel

What does it actually means?

And do you guys have a recommendation for learning yii framework other than Larry Ullman's?
I really need a tutorial.

Thanks.