All of those can be found in the datasheet.
Ext. Clock; - true square wave signal? Applied to which pin?
Doesn't matter. Applied to XTAL1.
Ext. Low-Freq. Crystal - no idea how this varies from the bottom one.
32Khz. Those other frequencies (typically less than 1Mhz) also work.
Full-swing Crystal - no idea how this varies from the bottom one.
High drive power. Typically for high frequency crystals or noisy environment. More current / power consumption.
Ext. Crystal Osc. - OK, I get this, but have never seen anyone suggest one other than the 16 mhz crystals. Some of the chips are rated for 20mhz, do you just use a 20mhz crystal to achieve that?
Yes, you can use other frequencies as well - from DC to whatever specified. Overclocking is also possible - I have ran them at 32Mhz no problem.