In Britain (there's already loads for definations for the way Australians use this word), this word generally means a young male. However in modern Britain this word has come to mean someone who engages in typical testosterone-driven behaviour such as drinking, sport and having a laugh with mates, sometimes harmless, sometimes obnoxious or even worse antisocial. There was once the phrase "Jack-the-Lad" (a rogue), "laddie" has long been part of the Scottish dialect, and in the 90s something called "Lad-culture" arose, where the aforementioned behaviour was celebrated. Girls who behaved in a similar way were called "Laddettes". There is also the adjective "laddish".
This young lad was killed in a bike accident recently

He's one of the lads, they love their football

On Friday nights the town centre's full of young lads getting drunk
by ManofG March 5, 2009
A British & Irish word used to describe a male aged between 9-30 years old or depending what way you use the word it can also be from 0-9 years old
"Where having a lads night this friday you up for it?"

"Those young lads are up to mischiff"

"My wife just gave birth to a wee lad"
by Harry69 July 6, 2007
A young Australian man/boy, generally between the ages of 12 and 25, who, due to a lack of any real parenting or discipline, lack the social aptitude for any real friendly connection or meaningful relationships.

As they find it difficult to attract real people these individuals gather in large groups and prey on the small and the weak in an attempt to hide their inherent fear of their own weakness.

Easily distinguishable by their nautica, saucony, and other sporting brand clothing (ironic in that even the healthiest lad pulls enough cones and smokes enough cigarettes that even light exercise such as running from police and beating down 8 year olds with their mates tires them out); not to mention various other ridiculous clothing and hair choices such as the famed nike TN's, which take many dole payments to acquire, the nautica white cap, and, worst of all, the rats tail, which has in actual fact been proven to cause severe mental disability.

the lads female counterparts, known as "lasses" (luminary I know), are generally very similar to lads in the respect that their parents mistakes create a girl who, with no self respect or ability, allow these lads (many at the same time), to stick them with their miniscule, unprotected excuses for penis's.
this in turn creates a new generation who, with their teen parents unable to provide them with even the most basic education in any academic or moral sense, turn into even worse versions of the preceding generation.

this is a vicious cycle as this causes a rapid degeneration of social behaviour in sydneys western suburbs.

As already mentioned, lads are known to roam in large groups and single out those who they sense are weaker then they are (with 10 of their mates), this list of weaker people involves women (though in many cases they still get their asses kicked), elderly people (who still can sometimes successfully fight back), children (same goes), and the mentally and physically handicapped (im sure you can guess what this caption alludes to by now =P).

They can usually be found in three places, namely any train station, as this proximity to transportation allows them to mobilize to other towns for parties or gang bash/rapes, on the trains themselves, where they amuse themselves by playing rave music on their phones, attempting to pick up, and tagging, and finally, at every damn party i seem to go to, where they insist on fighting everyone.

it is unknown from whence the lads originated, however, in a few short years they have spread from the western suburbs of sydney to lithgow in the upper blue mountains. if this trend continues it could turn into a countrywide epidemic of massive proportions, but as soon as the NSW government passes laws legitimizing their eradication as vermin the issue will be quickly resolved.

Or so we can only hope.
words cannot describe just how massively lads fail
by Sir Badass Killington July 13, 2009
A few things that describes 'Ladism' in Australia:

Nautica, Polo, Canterbury, CCC, Saucony, Nike-Fit/Dri-Fit, Nike TN, Champion, Everlast, Adidas, Puma, Lonsdale London, Pills, Winfield Blues, Marijuana, Graffiti, Raves, Gabber, Alcohol, Fighting, Crews (example: ALK - All Line Killers), Bumbag, In-Sport, Spitting, Acting Hard, Town Hall, Maccas, Shopping Centres, Pig-Latin, Eshays, Hardstyle, Rap
Lad 1: Wanna come for a sesh?
Lad 2: Fucken oath brah, eshays adlays.
by Town Hall August 18, 2008
Slang term for those Australian gangsters who they own the world. Easily recognisable by their fair skin, WHITE nike dri-fit hats (thats a must), polo ralph lauren, nautica, tommy hilfiger tshirts or sometimes nike tshirts. They also have those stupid baggy Everlast hoodies. They wear Nike shorts or tracksuit pants and pimp themselves up by wearing Adidas Bounce, or Nike Tunez shoes.
Lad 1: Oh yeah bra, lets bash up someone bra! Oh yeah, you like my new dri fit bra?

Lad 2: Fuck yeah! You're real lad bra!
by Da Barefoot 79 August 21, 2008
See that group of skinny kids wearing trackies, polo shirts, bum bag around their shoulders, shit hats with the brim turn up and sporting some of the worst haircuts in the history of mankind? Well there known as "lads'. Believed to have originated in the western suburbs of sydney, they have grown to annoy and harass the wider population They typically travel in packs to avoid being ridiculed for their dress sense. The use of 'pig latin' shows an inability to create any of their own slang. The typical lad has the intelligence of a small rodent and in some cases that of a pigeon. As a result of this lack of brain power their preferred pass time is to steal and graffiti. It is believed by top scientists that it is also a "trial by fire". The native dance of the Lad is called "Gabba" which kind of looks like their walking on the spot except heaps more messed up. Lads are hated throughout society and as a result are extremely aggressive and will try to start fights with anyone. No reason is too stupid or small for a group of lads to try and and take on one kid. Lads will remain in our society unless we group together, as a community, and remove them from our shopping centres and schools and into the history books to be forgotten for all of time.
Pig latin - "eshay adlay" (hes lad)
"WHAT CUNT!? YOU WANNA GO ME?!" the typical reaction when looking at lad or bumping into them accidentally
by lew-bird April 3, 2011
Its Australian term which means a boy at highschool who thinks they are really cool but the only people who think they are cool are other lads.
They generally wear white or blue polo hats with the bill popped up and Polo shirts with the collar up.
Dude 1: dude check out that lad, fuck look at him he thinks he's so cool.
Dude 2: Cha!
by 69Hangman69 April 10, 2006