By sending a kiss you are kind of flirting with that member and showing you're interested.
If you view a members profile at the bottom there will be a button saying 'send a kiss' - if you press on that it will open a new window where you can choose the type of kiss you would like to send.
On your homepage theres a Statistics section where you can see which members have sent you a kiss. Just click the link for the option: 'users who sent me a kiss' to see users list with which kisses they sent.
Yes. You can see this on your homepage stats section where there's a link for "users I sent a kiss". Click it to see a list of users with kisses you sent to them.
You can send as many kisses as you like - only if you're in someone's Black list your kisses won't be delivered.