MicroSD (and SD) cards have a class rating. Class 2 is the slowest (2MB/s) and Class 6 is currently the fastest (6MB/s).

For "normal" stuff on a smartphone, a Class 4 (4MB/s) will be fine, but if you plan to make a lot of photos/video, invest in class 6.

Generic cards are usually fine, but I'd recommend sticking with the better brands (Sandisk, Kingston, Patriot).

Also beware of really cheap cards on Ebay, some are fakes that do NOT deliver the promised size.