Equipment are a vital part of any unit's growth. In fact, they tend to account for about half of a unit's attributes, as well as covering various resistances.
Every job defines what equipment it uses, consisting of 6 pieces per job level. In order for a job to level up, it must equip all 6 pieces, at which point the equipment is absorbed and a new job level is available.
Note: Equipment is not the same thing as gear. Equipment is permanently applied (and is permanently used up), whereas gear can be moved between units to add additional stats (and is not permanently bound to a unit).
There are 2 primary types of equipment, standard equipment items and equipment that requires diagrams. Standard equipment items are either dropped as single items or as fragments of a single item, that when merged together form the entire equipment. Equipment that requires diagrams are assembled from the diagram (which can also be dropped as single items or as fragments which must be merged together) and also a set of other equipment items to complete the recipe. The number of fragments required to assemble an item is proportional to the rarity level of the equipment (i.e. no border, bronze, silver or gold border).
Job Specific Equipment
Most equipment items used by units are generic and are used by many different jobs. However is some equipment which is specific only to a single job. Every standard job has a special job-specific item (which is a diagram based equipment) which is only used by that job for the first slot for job levels 9-11. Enchanted jobs use that same job-specific diagram in addition to a special one for the enchanted job. Some special units with unique jobs (usually free collaboration units) may also have other special job-specific items which are used for lower job levels (these are typically not diagram based).
Time Sensitive Equipment
Job+ upgrades use specialized equipment only acquired via the job+ event for all 6 slots of job level 11 of the job. These items (as well as the corresponding job tokens) can only be acquired when the event is available so it is important to get these when they are available, even if you aren't ready to use them yet. These items can't be replaced with omnicrystal equipment.
Limited collaboration units (and free collaboration units) often use special job-specific equipment which is only available during their events for their unique jobs. It's important to get this before the event ends, otherwise you will be reliant on omnicrystal equipment to replace it.
Most equipment can be obtained via farming story quests. When you need a specific piece of equipment for a unit, it should give you a list of quests to obtain that equipment item. Some quests are better for specific equipment drops than others. Generally quests that drop less other items are preferred for farming a specific piece of a equipment. You can check the Best Places to Farm for a list of story quests that may be the best options for common non-job specific equipment pieces. You can also search the Alchemist Code Database: Items section to find where specific equipment pieces drop.
Limited Time Events
In addition to story quests, there are often good spots to farm specific equipment in limited time quests. Often higher level quests drop a useful selection of the higher level equipment items and may be an efficient place to farm them. If you have units which are specific to a collaboration, their job specific items are typically only available to farm within their limited time events. There is often quite a lot of job equipment given as rewards for Box Event summons as well.
The main shops are often a good place to buy diagrams of job specific equipment or also shards of general equipment for zeni. They refresh 5 times a day and it's a good idea to pre-buy diagrams for units you may one day wish to raise. They also sell some equipment for gems but this is less cost efficient to purchase and rarely a good idea. Sometimes the secret shop also has temporary good deals on equipment for zeni, and the Adventurer's Shop has a regular stock of lower level equipment items for zeni, such as great lances and star staves. Typically it is not cost efficient to buy equipment for gems in the secret shops, but occasionally there are cost efficient bundles, and for newer players who obtain limited units, it may sometimes be easier to purchase their job specific equipment for gems rather than trying to farm quests beyond their skill level. In addition you can buy equipment from some coin exchanges, such as the Multi Coin Exchange, Arena Coin Exchange and Veda Coin Exchange.
In addition to standard equipment, there are 2 special kinds of wildcard equipment available:
(Replaces equipment shard pieces)
(Replaces equipment diagram pieces)
These items can be used as replacements for standard equipment if you do not have enough of the item. These are primarily used to replace specialized equipment only available during limited collaboration events. Note: These items can't be used to replace job+ upgrade equipment.
Omnicrystal equipment are sometimes available in bundles, special coin shops, or as rewards or compensation of various type. There is no permanent way to acquire them.
Equipment can also be enhanced, raising their stats to at most 50% of their original values, at max rank (5 stars). It is important to note that when a job levels up, all enhancements are lost. Because of this you should never enhance equipment before job level 11. You also must be player level 60 to enhance equipment.
Enhancing equipment is very expensive. The zeni cost scales with the rarity of the equipment item enhanced. A gold-bordered equipment costs 504k zeni and 1680 points of enhancement materials to fully enhance, silver-bordered equipment costs 201,600 zeni and 1120 points of enhancement materials to fully enhance.
Only the active job benefits from the stats gained from enhanced equipment so you should never enhance equipment for a job you don't use.
To enhance equipment you can use alchemia pots. These can be purchased for zeni from shops or acquired as rewards through various game events. You can also farm these in the Reclaiming the Alchemia Pot event, however this is usually an inefficient use of AP so it is better to buy them from shops. You can also use other excess materials for enhancing but you should take care never to accidentally use equipment or other items for enhancing you might actually need later.