The simplest, most basic way to edit is with the Selection tool. Use the Selection tool to shorten or lengthen the end points of clips and to move them around in the Timeline. Be careful not to overwrite other clips when using the Selection tool, and be aware that the Selection tool will leave gaps between clips in the Timeline.

To trim a clip with the Selection tool, click-and-drag the start- or end-points to shorten or lengthen it. 

Use the Razor tool to split a whole clip into smaller portions. Splitting clips allows for chunks of a clip to be moved and edited separately from others.

To split a clip with the Razor tool, find a desired spot on a clip in the Timeline and click it. This will split the clip and created two separate pieces. Clips can be split multiple times, so repeat as desired. The split clips can now be moved independently of each other with the Selection tool.

Using the Selection tool to move clips can leave gaps or cause clips to overwrite each other. When the Ripple Edit tool is used to lengthen or shorten the end of a video clip, any clips to the right in the Timeline will be pushed along without causing any overwriting. For example, if you realize you need to edit a few seconds out from the start of your project, use the Ripple Edit tool to cut that time out; all subsequent clips will adjust automatically to cover those lost seconds. In the bottom-right of the Ripple Edit image, notice that the adjacent clip has been pushed down the Timeline.

To shorten/extend a clip with the Ripple Edit tool, click-and-drag an end point. Upon release, the clip will be shortened/extended and the subsequent clips will shift left or right in the Timeline.

A Roll Edit lengthens one clip while simultaneously shortening the clip immediately adjacent to it. This will cut the adjacent clip and prevent the overall project from gaining or losing time. In the bottom image to the right, notice the adjacent clip has not shifted in the Timeline.

To shorten/extend a clip with the Roll Edit tool, click-and-drag an end point. Upon release, the clip will be shortened/lengthened and the adjacent clip will be trimmed.

If the length of a clip is correct, but the portion of the clip thats visible isnt quite right, use the Slip tool to change the part of the clip thats shown without changing its length.

To change the timing of a clip with the Slip tool, click-and-drag left or right on the clip. The endpoint time signature in the bottom-left corner of the Program panel will change depending on how far the clip is moved.

The following techniques arent based on a tool. 

To swap the positioning of clips in the Timeline, click-and-drag a clip while holding Command and move it to the new desired location.

To insert a clip between two others in the Timeline without overwriting, hold Command+Option (Mac) or CTRL+Alt (Windows) and then click-and-drag a clip between others in the Timeline.

To overwrite a clip, follow the steps above but click the Overwrite button instead.

A Replace Edit will cut a new clip to be the exact same length as the clip being replaced in the Timeline and drop it in the same location. Use a Replace Edit when well-timed edits have already been made, but a different piece of footage is desired instead.

To replace a clip in the Timeline, click-and-drag the new clip from the Project panel while holding the Alt/Option key and place it on top of the old clip in the Timeline. A green circle with a plus sign will appear when the replacement is ready.