Enlargers are all pretty similar, as their function dictates their design. To begin with, there is a baseboard with a column (single or sometimes dual) attached. The baseboard may be replaced by a full stand, and occasionally the column is attached directly to a wall. The column supports the enlarger itself. The enlarger can be moved up and down the column, by means of a geared crank. Some larger models use a motor drive for large movements, with a crank or dial for fine movements. The enlarger is counterbalanced on the column, so movement should be easy, and the unit should stay where you set it, without necessarily having to lock it down. The crank is most often on the right side, towards the rear.
The enlarging head has several parts. The lamp house, or light source is at the top. This may be one of three different types. The first, and most common, is the condenser lamp house. The light source is an incandescent bulb, and the light passes through a series of lenses, or condensers, which then projects down (out). This type produces a very crisp image. Condenser enlargers often have an opening, or drawer at the bottom of the lamp house, above the negative and lens stages, for inserting filters.
The second type of head is the diffusion, or cold light head. This uses a florescent tube or tubes, and the light passes through a diffuser, or a piece of translucent glass or plastic to produce even illumination. This produces a somewhat softer image that a condenser enlarger. They are now sold as variable-contrast enlargers, with filtration for manipulating contrast built in.
The third type of lamp house is the dichroic, or color head. These use a tungsten lamp (usually), and have magenta, cyan, and yellow filtration available. They can be used for black and white printing, but require a bit of knowledge of CMY color.
Below the lamp house is the negative stage. There is usually some type of lever which tilts the lamp house back at the lens stage so you can insert and remove a negative carrier, which holds the negative. This carrier inserts into the lens stage one way, either resting in a cutout, or aligning with pins or bars, and they do have an up and down side. The carriers usually open clamshell-wise. The negative is inserted either into or against a set of guide pins, emulsion-side down.
Below the negative stage is the lens stage, attached to the negative stage by a geared column and a bellows, so that it can be move up and down to focus. The lens stage holds the mounted lens. The focusing knob is usually on the right towards the front.
Enlarging lenses are much like camera lenses in that they have an aperture range, change by rotating the aperture ring, which controls the amount of light passing through. They are also matched by focal length to different film formats, with a 50mm for 35mm format being normal, around 80mm for medium format, and around 120-150mm for 4 x 5 format.
Below the lens, mounted to the lens support column, is a filter stage, for using under-lens filters. This can usually be rotated out of the way when not using filters, and may be able to be adjusted up and down to accomodate different lenses.
Enlargers may be able to handle different formats. There are models which will only print 35mm, others which will print up to 6 x 7, and others up to 4 x 5, 8 x 10, or larger. They will require adjustment of the lamp house, different negative carriers, and a change of lens.