metronome terminology

No - that's not quite right. It depends on how you choose to set the metronome.

A triplet group is 3 notes played in the time normally taken by two notes of the same type. So three sixteenth note triplets would be played in the time normally taken by two sixteenth notes or one eighth note. If your metronome is currently counting eighth note beats then those three sixteenth note triplets will be played under one beep of the metronome. If your metronome is currently counting quarter note beats, then the three sixteenth note triplets would occupy exactly half the time between beeps.

ah - not sure i understand fully, but i do now know i have been using my metronome wrong.
i have just been setting it to a set number beats per minute, and practice my picking to that. At the minute i am playing at 151 bpm and picking 3 notes for each beep (just through the major scale), which i thought was triplets at 150 (not terrible fast but hey, ill get there) - but there is no emphasis on any of the beeps, they are all the same.

ill track down a different internet metronome and see if i can set it to emphasise on of the beeps, hopefully itll begin to get a bit clearer then. thanks. Al

If you're not actually referring to any written music, but just using it for speed training, then what you're doing is fine. Without a time signature specifying the beat unit, (quarter note, eighth note etc) then what you're doing is playing 3 notes per beep and it doesn't matter whether you call them quarter, eighth or sixteenth notes, etc. they're just notes and there's no need to search a new metronome that emphasises certain beeps (although it would be handy when using it to keep time in real music, which does have a specified beat)
Edit - a bit more...
Let's say the metronome is beeping away at 150 bpm.
If you call those beeps quarter note beats and you play three notes per beat, then you're playing eighth note triplets.
If you call the metronome beats eighth note beats and you play three notes per beat then you're playing sixteenth note triplets. Same sound, same speed, different name.
It's all relative and it's up to you what you want to call the beat unit - that's if you want to call it anything at all.

thanks fretsource, i really do appreciate the detailed response.

i am confused about one more thing now though.
i dont quite understand why calling the beeps quater notes would give me eighth note triplets, - as to me quater means 4 - why are they not called quater note triplets??.
also, if the beeps are eighth notes, why would it then be 16th note triplets instead of eighth note triplets??

im sure that i am missing something quite obvious about the naming of these triplets, but i just need a nudge to see it. :D

i dont quite understand why calling the beeps quater notes would give me eighth note triplets, - as to me quater means 4 - why are they not called quater note triplets??.
also, if the beeps are eighth notes, why would it then be 16th note triplets instead of eighth note triplets??

Normally a quarter note equals TWO eighth notes. If you were playing two notes for every quarter note beep of the metronome, then you would say that you're playing eighth notes. But if you play THREE notes for every quarter note you can't say that you're playing eighth notes. Your notes are shorter than eighth notes (3 in the time of 2) so you have to say you're playing TRIPLET eighth notes, rather than straightforward eighth notes.

The same applies if your metronome is counting eighth note beats. Instead of you playing TWO sixteenth notes per beep, you're playing THREE notes. You can't call them sixteenth notes as they're shorter than true sixteenth notes (again 3 in the time of 2) - so you must call them TRIPLET SIXTEENTH notes to distinguish them from true sixteenth notes.

thanks fretsource. that was a big help :D