All cylinders are used for lifting of heavy loads, plus all cylinders can be used horizontally for pushing. In addition to these functions, "hollow" (also called "center hole") jacks can be used for pulling. And "Lock nut" jacks can be used for long periods (days and months) of load holding. Cylinder jacks are divided into several types:

Standard (also called "General Purpose")

These hydraulic cylinders are used for versatile lifting & pushing.

These hydraulic cylinders are used for lifting under low clearance. Low profile cylinders include "flat" jacks and "pancake" jacks. A jack labelled "low profile" has the same shape as a standard cylinder; it is just shorter. A jack labelled "flat" jack has more of an oval shape and a "pancake" jack has a standard cylindrical shape but with a larger diameter making look more like a pancake. Usually the flat jacks and the pancake jacks have the lowest overall height as low as 1.28 in consequently these are also referred to as "Ultra-low profile" jacks.

Regular size cylinders are tall & thin.

Low profile cylinders are short & stubby. 

These hydraulic cylinders are more lightweight for easy transport and they have excellent corrosion resistance.

Steel is tougher against fatigue. It is employed under toughest working conditions. Aluminum is more lightweight and its non-corrosive body holds up better to weather. It is not for use under constant motion (for example in production) and under constant use wears sooner.

Lock Nut (also called "Locking Collar")

These hydraulic cylinders are used for lifting & holding load over extended period of time. The plunger has a locking collar (nut) for mechanical securing of load when extended periods of non-hydraulic support are required meaning the plunger is threaded and has a nut. Once you lift the plunger out of the cylinder, you can screw down the "locking" nut which will permanently hold the load without creeping.

Hollow (also called "Center Hole")

In addition to general purpose lifting or horizontal pushing, the hollow cylinder is used for pulling. The jack has a center hole that runs from the top of the ram through the bottom which can be used to pull a chain or rope through the center hole to tug, pull and create tension. This is used, for example, to tighten the wire ropes on suspended bridges.

The plunger is the part of the jack that rises out of the housing (cylinder body). The plunger is also referred to as a "Piston" or "Ram". 

The "stroke" means how high the jack is able to lift from the retracted position. The "stroke" it is the height difference between a retracted and extended jack.

The oil flow also called "flow rate" is the amount of oil, the pump delivers to the cylinder (... for hand pumps with one hand stroke, ... for electric and air pumps per minute). The larger the flow rate, the faster the cylinder will rise.

Some pumps are equipped with double speed: With the cylinder unloaded, the pump delivers a high volume of oil for fast cylinder speed. Once the load is engaged, the pump automatically switches to the slower high pressure lifting stage.

How many hand strokes are required for a hand pump to raise a cylinder?

Hand strokes = Cylinder oil capacity / Oil volume per stroke

The "collapsed height" is the height of the cylinder when fully retracted. The "extended height" is the height of the cylinder at full stroke.

The width of any cylinder is given as the "outside diameter". Flat cylinders have more of an oval shape. The "outside diameter" is measured at the widest outside-to-outside point. The hollow cylinder has a center hole. The "inside diameter" gives the width of the hole.

The cylinder effective area is the piston's surface area that comes in contact with the load.

The "usable oil capacity" in the pump must always be larger than the oil capacity in the cylinder or the cylinder will not lift to the maximum extended height. Also take into account that the hoses and fittings will have to be filled with oil when you pressurize the system. Consequently make sure you have plenty more oil in the pump than the cylinder requires, especially if you are operating long hoses and possibly multiple hoses and accessories. As a rule of thumb please allow 20 percent additional usable oil volume in the pump than the cylinder(s) requires.

Note that the pump has a larger oil tank than its usable oil capacity. Often the usable oil capacity is only 75 percent of the tanks volume so you may find a 1 gallon tank on a pump with only 0.75 gallon usable oil. The reason is that the oil requires some breathing room especially when it is flowing back into the pump under high pressure.

General purpose cylinders are limited to the number of raise and retract cycles they can sustain before maintenance considerations become an important factor.

Low-cycle: Infrequent use of cylinder. Mainly used in construction and maintenance.

High-cycle: Continuous up and down movement of plunger/piston. Used in production. High duty is generally considered as greater than 2,500 life time cycles. High duty cylinders (also known as industrial grade) are fabricated of rugged, resilient materials. High-cycle cylinders are often used as components in manufacturing machines.

Aluminum jacks cannot be used for high cycle applications. As a rule of thumb, pumps require 20% more oil volume than jack(s) for low-cycle applications and double the oil volume of jack(s) for high-cycle applications.

Cylinders are always considered "Low-cycle" cylinders unless otherwise labelled as "High-cycle" cylinders.

Cylinders are always considered "Single-acting" cylinders unless otherwise labelled as "Double-acting" cylinders.

Single Acting - Oil is pumped one way into jack to extend/raise piston out of cylinder – Either steel spring in jack OR weight of lowering load retracts/pulls down piston and pushes oil back into pump. Jacks are ideal for lifting, holding, pushing & pulling.

Double Acting - Oil is pumped two ways: (1) into jack to extend/raise piston out of cylinder and (2) out of jack to retract/push down piston and return oil back into pump. Jacks are ideal for lifting and controlled descent.

Single Speed - The pump operates the jack at one and the same consistent speed.

Double Speed - With the cylinder unloaded, pump delivers a high volume of oil for fast cylinder speed. Once the load is engaged, the pump automatically switches to the slower high-pressure lifting stage.

SCFM stands for Standard Cubic Feet per Minute. It denotes how much air volume (measured in cubic feet) is needed per minute from an air supply (compressor /shop air) to operate an air over hydraulic pump.

NPTF stands for National Pipe Thread Fine. It denotes the inside diameter of a fitting or hydraulic hose. The standard sizes are 3/8 inch NPTF (also referred to as fast flow) or 1/4 inch NPTF.