Blur filters are named much like they work or affect your video. They blur or defocus your video in various ways. Used to extremes, they can create an interesting image of colors and shapes . You might also want to use them in conjunction with other filters, such as a 4- or 8-Point Matte Filter, to hide a person's identity, for example. In this case, you'd put identical clips on top of each other, apply the blur to the lower clip, and use the Garbage Matte to reveal only the area you want to blur on the upper clip.
Gaussian Blur blurs the whole frame of a clip's video. You can blur one or all of the color and alpha channels together or separately, creating a different look to the blur. The pop-up menu it contains allows you to choose which channel to blur. The Radius slider allows you to select the amount of blurring you add to the clip. The more you move the slider to the right, the more radical the blur. Setting a keyframe at 0 and then setting one later in your clip at a higher level creates the illusion of a rack focus, for example. A rack focus is where you shoot two people or objects, and one is behind the other, in the distance. You can change focus during the shot to bring one or the other into focus. This one is probably the most used of the group of Blur filters. It's also often used as a way to blur a face using a Garbage Matte technique, much like you did with the clip of Cap looking at the stars. However, you use the same video on both layers and add the Blur and the Matte to one of them. Used at an extreme setting, a Gaussian Blur filter makes interesting backgrounds, because they become very abstract. Used with slowed-down footage, they can be very effective for title backgrounds or fills.
Radial Blur creates the effect that the image is swirling around a selectable point (the Center button). The Angle control adjusts the amount of the blur. The Steps slider smoothes the blur. You can specify the center point in the frame about which the blur rotates by clicking the Center button and then clicking any area of the frame, creating an anchor point to rotate it around.
Wind Blur creates the effect that the video is being blown by wind. You can adjust the direction in which the blur travels using the Angle control and specify the distance between each increment of blur with the Amount slider. The Steps slider determines how smooth the blur appears.
Zoom Blur creates the effect that the image is moving toward or away from you. A pop-up menu allows you to select whether the blur moves in or out. The Radius slider determines the distance between increments of blur, and the Steps slider determines the smoothness.