When you look up some verbs within the dictionary they have a suffix entry immediately below them.........

Examples:

görmek (to see)
before the first entry it has ",-ür"

gelmek (to come)
before the first entry it has ",-ir"

vurmak (to kill)
before the first entry it has ",-ur"

What does this suffix indicate and why do only some verbs have this entry???

(r) is the affix of simple present tense and other letters are stickers for them...it adds the word simple present...got?

Why have you added the plural suffix twice in your second example? Is that what you meant to write "havlarlar"

Hi bod,
I think the reason is that the aorist tense has too many rules on adding the correct suffix. The number of syllables and the last consonant has an affect. So probably it is to make clear which suffix to add.

http://www.turkishclass.com/grammar_presentSimpleTense.htm

I think there is no information for havlamak becuase here you simply add -r > havlar

Havlarlar is correct but there is a problem. Havla+ r+ lar : verb stem + aorist tense suffix + 3.th person plural suffix
The problem with havlarlar is that we use the singular with animals and objects. If the animals have individual personalities (if their names are known to the listener and speaker) than both singular and plural would be acceptable as it is for human.
In general, for human we can use singular or plural very conjugation for third plural person.

Thanks Erdinç.

But it is still not clear to me why some verbs have '-ir' or '-ur' or '-der' at the top of their dictionary entry and some do not :-S

Example with '-ir'
vermek

Example with '-ur'
vurmak

Example with '-der'
gitmek

Yes it is right. The rules above could be applied to the ones which have an entry. This means you could find the correct suffix on your own by following the complicated rules for this tense but you would lose some time.