There are three major methods for purchasing your clothes: off the rack (OTR), made to measure (MTM), and custom. There is also another method that you will see frequently called bespoke which we will also discuss.
The first and most important thing to understand here is this: THERE IS NO GOVERNING BODY THAT DETERMINES EXACTLY WHAT EACH ONE IS! There is a fairly general consensus, and with technology the lines are blurred between things more and more.
1. OTR:

2. MTM:
Made to measure is the next step along the purchasing scale. A suit that is MTM is going to have some distinct advantages. The customer's measurements are taken, and then the tailor will adjust the cuts and creation of the garment based on these measurements. They still use a stock sized pattern, but adjust the garment before finishing it. **easy analogy pt 2 - paint the picture (no color code), similar but more individual
3. CUSTOM:
Custom and MTM walk a very fine line. Because of technology, a MTM garment can begin to sound eerily like custom. Custom is typically done with a unique pattern being made for the individual using their measurements and usually pictures. Then the garment is put together and finished per the specifications of the customer. The key difference here is that custom starts from the measurements - not being adjusted after the pattern. **easy analogy pt 3 - you tell the artist what you want the painting to be, then they create
4. BESPOKE:
This is the hardest one to differentiate. When you read the idea of bespoke tailoring, it is almost a dead on to what I consider to be custom. Literally meaning that the garment has "been spoken for," this is a truly unique garment being made to the exact specifications of a customer and their measurements. It is typically accepted that the major point of difference is that a bespoke garment requires multiple fittings while being created. Unique pattern + multiple fittings (prior to finishing)= bespoke.
Sooooooooooo......
Which is right for you? Some of that will come down to your preferred price-point and when you need the clothing. Understandably, each tier is typically more expensive (though depending on fabric, brand, etc that isn't always true) so a quick reference guide and please understand this is a general guide NOT the rule all the time:
OTR: price - $100 and up time needed - up to a few days for alterations
MTM: price - $400 and up time needed - 2-4 weeks
Custom: price - $600 and up time needed - typical 4-6 weeks (some offer 2-4)
Bespoke: price - $1500 and up time needed - varies greatly sometime measured in months
Ask around, do your homework, and understand that like many things in life - you get what you pay for.
Sorry this one was longer guys! Hopefully it was good info though and something worth looking at and sharing. Keep reading and sharing!