Genealogical publications are protected by copyright . Although data often comes from public archives, searching, interpreting, collecting, selecting and organizing that data produces a unique work. Similarly the publications on Genealogy Online. So you may not just take over everything from others! But you may use someone else's work for your own research. You may therefore extract certain data from it and include it in your own family tree, as long as you refer to the work where you retrieved that data and your own work does not look too much like that other work.
Advice
Highly recommended : Indicate whose data you have copied, ideally also his / her original source!
Strongly recommended : Request permission to copy data or at least inform the author, chances are that the author gives permission, the contact often leads to more data exchange!
Strongly recommended : Only use the data after you have checked it, preferably at the source (the archive)!
Recommended : State from where (website / book / magazine address) you have copied the data, this way an interested person can look further at the source!
Genealogical publications are copyright protected. Although data is often retrieved from public archives, the searching, interpreting, collecting, selecting and sorting of the data results in a unique product. Copyright protected work may not simply be copied or republished.
Please stick to the following rules
Request permission to copy data or at least inform the author, chances are that the author gives permission, often the contact also leads to more exchange of data.
Do not use this data until you have checked it, preferably at the source (the archives).
State from whom you have copied the data and ideally also his/her original source.