If you walk up to a Vendor and say Job, the vendor may ask you to make some items for them. The items requested depend on the vendor and what your skills are. Not all vendors will have jobs for you. If there is nothing that you can do for them they will tell you, "I don't have any work for you right now."
If you can make something for the vendor you will be asked to accept or decline the job. If you accept the job you will be handed a bag and will immediately begin work. Be careful if there is more than one vendor close by, both may ask for a job and only one will be accepted. If you are unsure who you got the job from, single click on the bag and it will tell you who it is for. When you finish making the item you simply need to give the bag to the vendor and get paid. The money you will make is based on your skill. To determine how much money you will make per item divide your skill level by 5 and round down to the nearist whole gold piece. For Example, If you have 92.3% Blacksmithing skill you will make 18 gold per item. (92.3/5=18.46=18 gold per item). You will also get some skill gain from jobbing. The rate of skill gain is unknown at this time.
There is a jobbing limit of about 5000 gold per day per account. Once you've reached this limit, you won't be able to do any jobs again until the next day. When you attempt to job, the merchant will tell you to come back the next day. This is based on the gold value of the jobs you do, not the number, so the higher your skill in the jobbing skill, the quicker you'll reach the limit. (also note that this is different from the limited number of jobs that each merchant has. If the merchant says he doesn't have any work for you right now, that means that that particular merchant doesn't have any work available at the moment, but you may be able to do more jobs elsewhere. If he tells you to come back another day, though, then you've reached your daily limit.)
The 5000gp per day limit is based on a 24 hour period, it does not 'reset' at midnight. How you are limited can be a bit complicated to understand: its 5000 gold in any given 24 hour period. So say you job for a while and get 2000 gold, then take a break. 8 hours later, you job for the other 3000 gold. At that point, you won't be able to job for 16 hours, but when you do, you'll only be able to do 2000 gold worth of jobbing (and then the other 3000 8 hours after that.)