AGN:NYSE was acquired today by AbbVie Inc, and lets assume that they bought the whole 100% of the company, now my question is would the acquired company be delisted from stock exchange and "merged" into parent or what? Why does it or doesn't happen? What happens to stocks if it is "merged" or are they bought-out?
Acquisitions can be made for all stock, all cash, or a combination of both.
Prior to approval of the merger, the deal can fall apart or someone else can come in with a higher offer (see the recent offer by CVX to buy APC which was then beat by OXY).
After the acquisition is approved, at a later date it is finalized. On that date, the acquired company will cease to exist and its shareholders will have their shares replaced by whatever the merger agreement stipulates times the number of shares held.
So if A is taking over B and it's a 3 for 2 all stock deal, an owner of 100 shares of B ends up with 150 shares of A. If fractional shares are involved, B will receive the fractional shares in cash.
The company which has been acquired will normally be delisted (sometimes the combined company operates under the name of the company which got acquired). In the deal, the company buying will pay cash plus stock for the other company. The shareholders of the company which will cease to exist get that money and those shares in the new, combined company. So yes, they are delisted and the investors are merged in with the investors of the other company.