merger = two companies come together "permanently"for mutual gains or to reduce competition
acquisition = one company buys another company which may or may not be doingwell
takeover = same like "acquisition", but generally a company buysanother company which is not doing well or has gone bankrupt.
joint venture = two companies come together "temporarily" for mutualgains for a particular project/job. after the project/job is completed thejoint venture is dissolved.