There are many kinds of dot-TLD domains.
Is it ok to use them instead of using dot-com ?
Are there any big different?
That might depend upon the goals of your website, target customer lives where, speaks what language, in 5 years will you outgrow a ccTLD? If you are building a new site or buying a domain with hopes to sell for a profit in the future, might I suggest that you review comparable domain names once in a while. Visit
http://www.buydomains.com to review different TLDs and prices asked currently on the market. Also visit
http://www.dndatabase.com to look at actual sales. Price is one indication of what it takes to be a "good" domain name. Don't buy or register a domain with a hyphen, it's a kiss of death if you ever want to sell the site for big $$$. Hope this helps.