if you search this forum this question has been answered very well before i believe but here goes my best take:
"blanks" such as ready made tees (by alstyle, gildan, hanes etc) can be purchased in small or large quantities and sent to a printer for printing. this is the easiest and cheapest way to make t-shirts for small brands. it is also very reliable because the t-shirt blanks companies are very consistent. whether you personally like those blanks or not is a different argument, i'm just saying they tend to be of the same cut and quality. Another aspect of using blanks is that if you need x amount tomorrow, you can run out and buy them, or call up and orders some to be delivered the next day. in terms of customizing it, you can change the inside label to make it look better, or take out the label and print your logo inside hte neck etc.
"cut & sew" in regards to t-shirts, means the company has designed and manufactured their own t-shirt body from scratch. this can be quite expensive, require a litte technical know how, and requires the longer production time of making something from scratch (possibly overseas and then shipping it from there too), and also the risk that if the blanks come out bad you're totally screwed...and picn!k is right insomuch as the designs are often printed on cut & sew tees before the garment is completely assembled - which is why cut & sew allover prints often look cleaner.
you'll notice some brands switch from printing on blanks to making their own cut & sew tees when they reach a certain size perhaps or get investment. other brands do cut & sew off the bat.
Aside from "blank" tees and sweats (you can also buy plain mass produced crews, hoodies, and zip ups for example) pretty much any other garment is "cut & sew" because it has been made from scratch to that company's specs. there just aren't (good) blanks available for jeans or jackets or shirts that i know of...especially not ones available tomorrow...these things must be made from scratch usually...
for small t-shirt brands that print on readily available blanks, it is often quite a big step to jump into doing "cut & sew" collections because of the increased expense, the work involved, the stress of having to plan much further ahead due to scheduling in production turnaround times for manufacturing...and on the other hand it's more exciting because you get complete design control over your end product - if you want to make a t-shirt with an inside pocket, you can do that. if you want it to be dyed a certain color and enzyme washed for softness you can do that. if you want a certain weight of cotton you can specify that...if you want logo tape on the neck collar...you get my point...ditto for jeans, jackets, shirts, everything.