The simplest way to understand what a flat bar bike is to think of it as a sportier version of a traditional road bike. It simply has flat handlebars in place of drop down handlebars which you would see on normal bikes.
As a result of this flat bar road bikes are much sportier, they feature quicker handling and are generally lighter weight, their gearing is often also optimized for speed.

The lack of having drop down handlebars means that flat bar bikes are not quite as aggressive as road bikes, however, they do feature some of the same technology and performance elements as road bikes.
Flat bar bikes do provide great comfort and enhanced visibility as they have a more upright position. This makes them an ideal vehicle for recreational riding, fitness, fast commuting, competing and sport.
They are the sportier option of bikes. They share many similarities to endurance road bikes, with the overall aim of combining performance and comfort into one whole bike.
One of the main reasons for this is down to the dimensions of the bike, but it is also down to the use of a lightweight frame.
The tires on these types of bikes also allow for greater speed and efficiency when riding, while they also offer decent puncture protection from obstacles such as tram tracks and curbs.
Flat bar road bike tires are often around 28-32c wide, which is ever so slightly wider than a traditional road bike tire.
It is also worth noting that there is no suspension on flat bar road bikes as well as they are strictly used for tarmac, and suspension would only add more unnecessary weight to the bike.
The geometry of these bikes is also important, but so is the gearing which is optimized for flat roads.
The whole bike is optimized for flat roads, so these bikes are the perfect option to get you to and from work, as long as your work journey is also flat roads.
Are flat bar road bikes good?
Whether flat bar road bikes are good or not is a matter of perspective. If you like flat bar handlebars on your bikes then this type of bike will traditionally be more appealing to you than it may be to others.
However, road bikes do sometimes benefit from the drop down style of handlebar. This is not to say that they are better, it simply depends on your riding style and what you prefer in your bikes.
Drop down handlebars can provide some advantages if you want to go on longer rides, or if you want to ride with a group.
The bent shape of drop down handlebars will allow you to position your hands in multiple different ways. This means you can stay comfortable, able to change the position of your hands during your ride for better comfort.
Some of the positions you can use can be more aerodynamic for times when you may want to ride faster.
However, it is also perfectly okay for you to ride a road bike that has flat handlebars.
You will get the advantages that are provided by any bar that will give you a steady grip like on a mountain bike, which is good for giving beginner cyclists confidence.
However, there are also many riders who are not beginners who much prefer the more upright position or a flat bar road bike.
It really depends on what you seek personally in a bike, not only your preferred style of handle bar by aesthetic, but also how it positions your body on the bike, whether you prefer the upright position that a flat bar provides, or if you are someone who prefers the multiple comforts that a drop down bar can provide.
It is a totally personal decision, neither is better than the other really, it is simply a matter of perspective.
Can I put flat bars on a road bike?
If you have a normal road bike with a traditional drop down bar, you may consider transitioning it into a flat bar, while this may seem tempting, and doable, it can just be better to simply buy a flat bar road bike, straight out.
It is possible for you to transition the bar type of bike, however you would need a lot of materials, and it would take a great deal of time. On top of that it is very expensive to do.
To do this you would need to buy new brake levers and shifters and probably a new FD.
It is not a very hard conversion to do to any bike, however, due to the amount of new materials that you would need, which generally are not the cheapest of items, you could run into a lot of spending doing this.
To which the point is, it may actually end up being cheaper and more cost effective to just buy a whole new bike.
There is no need to spend that amount of money for something that will not change a great deal and even if it does, due to it being a home-make conversion there is also the possibility of a lot of going wrong with it.
Buying a new bike instead of just converting your current one may sound crazy, but it can work out much better financially and in terms of stability too, in the long run.
The cost of the brake levers, shifters, and new FD is nearly impossible to achieve for less than $200.
Obviously if you are unsure if you want to use this type of bike you may be skeptical about splurging out the money on a whole new bike.
But with the costs of the conversion, it can be more worthwhile to buy a new bike, at least if you do not like using a flat bar then you still have your old drop down bar bike.
It may be even more difficult to convert it back if you are not happy with the flat bar job.