If i didnt purchase HoTS when it is out? will i still be able to play with WoL?

no new units. no new story. obviously.

You can still play WoL, but probably without new updates.

but still can play multiplayer?

still can, but that time usually players will switch to HotS d.

i play at cybercafe....i dont understand the mechanics of expansion. how will cc update theirs?

it's quite simple.. just install HotS and there'll be another new icon for SC2 which is HotS just like how SC to BW or WC3 to FT.. Probably the new icon will have a more Zerg-y design to it.. Either way, the system will be able to play both and for CC dun have to worry cuz the SC2 client can DL for free..

heheh. looks like im purchasing this for the sake of the gamekey. their cds will be useless to me. lol