In today's digital age this is a tough question to answer, especially if you're starting out. For us 'old folks' who have been using film for a long time, coloured filters and ND filters were sometimes used for special effects or to bring dynamic range under control in the case of graduated ND filters. Gels were and are still used over lights. Today one can achieve these effects using software, HDR processing, etc. but it's really up to the photographer. There are those who prefer to 'do it all in post' and there are those who prefer to get as much of the shot 'in the can' as possible, and do final cleanups on the computer. It depends on where you're most comfortable - behind the camera or in front of the computer. Keep in mind that what isn't captured in the field is much harder to create later, so if you are planning to do a lot of post-processing, don't be stingy with your images while making photographs.
Again, this is a holdover from film. Someone used to shooting digital may make a thousand exposures a day. Someone who measures exposures in terms of 36-exposure films might make a hundred or two, and someone working with 4x5 or larger film plates might make only a few exposures, or none.