what are the basic things while we purchase a domain,,,,, i wanna buy few domain but dont know the basic things mean whom should i preffer mean domain with keywords or other,,,,,,,,,,
Relevant to the content... short as possible... relevant TLD (ie if you operate primarily in one country, consider a ccTLD for that country).
I'd say it depends on the purpose - if you're looking to create a strong brand, go for something memorable. If you're not worried about the brand, get something w/ the KW in it. I'd also go .com, .net or .org in that order, nothing else unless it is country specific.