IMPEDANCE the measurement of your speaker coil with an ohms meter it doesn't tell the power handling but it affects what the speaker will draw from the amp  

  DI direct input a kind of connection your mixer man will know about via a cannon plug connected to your instrument spkr some amps have this built in it means you get you own channel on the mix desk  

100VOLT LINE  what it says 100 volt speaker out for those perimeter fence horns the reason is you can have really long wires without problems ;ordinary spkrs need a transformer to isolate them from the live at each cab  

 transistors and MOSFETS most common is" the dollar transistor"2n3055 its an npn in an oval metal case expect 50 watts from two of these rca is the recommended make a similar slightly better output transistor is the 2sc3773 this one will cost you about £2.50 these run best at 8 ohms MOSFETS are reliable and give more power at lower impedance( witch is why you get car spkrs with 2 ohm coils )run at slightly lower voltage than transistors & more often used in pairs 2sj/sk for example flat pak and metal case better to use a powerful one than string a lot together expect to pay between 5 to £25 for a single replacement

cone types card surrounds for guitar because that stops the cone from moving suddenly and popping right out the material type for pa when every sound is mixed the rubber for home audio to get your full sound back

feedback when the sound from the spkrs is sufficiently loud enough to reach the input device be it micro vibrating the strings of the guitar  termed acoustical feedback ( feedback in an amp circuit is different thing) feed back is affected by acoustics of the hall and usually occurs at certain frequencies