Economists distinguish between five major kinds of unemployment, i.e., cyclical, frictional, structural, classical, and Marxian. (Another distinction, not discussed here, is between voluntary and involuntary unemployment.) Real-world unemployment may combine different types, while all five might exist at one time. The magnitude of each of these is difficult to measure, partly because they overlap and are thus hard to separate from each other. All but cyclical unemployment can be seen as existing at full employment, the level of employment and unemployment that represents the inflation barrier to demand-side growth.

Cyclical unemployment
This unemployment involves people being temporarily between jobs, searching for new ones; it is compatible with full employment. It is sometimes called search unemployment and can be voluntary. New entrants (such as graduating students) and re-entrants (such as former homemakers) can also suffer a spell of frictional unemployment.
Frictional unemployment exists because both jobs and workers are heterogenous, and a mismatch can result between the characterictics of supply and demand. Such a mismatch can be related to skills, payment, worktime, location, attitude, taste, and a multitude of other factors. Workers as well as employers accept a certain level of imperfection, risk or compromise, but usually not right away; they will invest some time and effort to find a better match. This is in fact beneficial to the economy since it results in a better allocation of resources. However, if the search takes too long and mismatches are too frequent, the economy suffers, since some work will not get done. Therefore, governments will seek ways to reduce unnecessary frictional unemployment.
Policies to reduce frictional unemployment include:
Frictional unemployment coincides with an equal number of vacancies. Numerically, it is therefore maximal when the labour market is in equilibrium. When for instance demand far exceeds supply, the frictionally unemployed will be few as they will get many job offers.
The imperfection of the labour market is sometimes graphically presented with a UV-curve, a hyperbolic or similarly shaped curve that shows a fixed relationship between the unemployment rate on one axis and the vacancy rate on the other. If the economy changes, the labour market will move along this curve. Factors that affect friction will shift the curve inwards or outwards. It is possible to derive this curve mathematically by aggregating (infinitely small) submarkets of the labour market, if it is assumed that these submarkets follow a probability distribution. Formulae have been derived for the Normal distribution; the latter has the hyperbolic UV-curve (U x V = c) as a special case.

educational advice;
schooling and training facilities;
information on available jobs and workers;
combatting prejudice (against certain workers, jobs or locations);
incentives and regulations (e.g. when the frictionally unemployed receive benefits);
relocation of industries and services;
facilities to increase availability and flexibility (e.g. daycare centers);
aid or grants to overcome a specific obstacle (e.g. if a handicapped worker is employed);
reduction of the gap between gross and net wages (e.g. by taxing consumption instead). Frictional unemployment
One kind of frictional unemployment is called wait unemployment: it refers to the effects of the existence of some sectors where employed workers are paid more than the market-clearing equilibrium wage. Not only does this restrict the amount of employment in the high-wage sector, but it attracts workers from other sectors who wait to try to get jobs there. The main problem with this theory is that such workers will likely "wait" while having jobs, so that they are not counted as unemployed. In Hollywood, for example, those who are waiting for acting jobs also wait on tables in restaurants for pay (while acting in "Equity Waiver" plays at night for no pay). However, these workers might be seen as underemployed (definition 1).
Another type of frictional unemployment is seasonal unemployment, where specific industries or occupations are characterized by seasonal work which may lead to unemployment. Examples include workers employed during farm harvest times or those working Winter jobs on the ski slopes or Summer jobs such as life-guarding at pools and agricultural labor.

Examples

Main article: Structural unemployment Hidden unemployment
In this case, like that of cyclical unemployment, the number of job-seekers exceeds the number of vacancies. However, the problem here is not aggregate demand failure. In this situation, real wages are higher than the market-equilibrium wage. In simple terms, institutions such as "the minimum wage" deter employers from hiring all of the available workers, because the cost would exceed the technologically-determined benefit of hiring them (the marginal product of labor). Some economists theorize that this type of unemployment can be reduced by increasing the flexibility of wages (e.g., abolishing minimum wages or employee protection), to make the labor market more like a financial market.

Marxian unemployment
In theory, it is possible to abolish cyclical unemployment by increasing the aggregate demand for products and workers. However, eventually the economy hits an "inflation barrier" imposed by the four other (supply-side) kinds of unemployment to the extent that they exist.
Some economists see the inflation barrier as corresponding to the natural rate of unemployment. The "natural" rate of unemployment is defined as the rate of unemployment that exists when the labour market is in equilibrium and there is pressure for neither rising inflation rates nor falling inflation rates [2]. More scientifically, this rate is sometimes referred to as the NAIRU or the Non-Accelerating Inflation Rate of Unemployment
No matter what its name, this means that if the unemployment rate gets "too low," inflation will get worse and worse (accelerate) in the absence of wage and price controls (incomes policies). Others simply see the possibility of inflation rising as the unemployment rate falls. This is the famous Phillips curve.
One of the major problems with the NAIRU theory is that no-one knows exactly what the NAIRU is (while it clearly changes over time). The margin of error can be quite high relative to the actual unemployment rate, making it hard to use the NAIRU in policy-making.
Another, normative, definition of full employment might be called the ideal unemployment rate. It would exclude all types of unemployment that represent forms of inefficiency. This type of "full employment" unemployment would correspond to only frictional unemployment (excluding that part encouraging the McJobs management strategy) and would thus be very low. However, it would be impossible to attain this full-employment target using only demand-side Keynesian stimulus without getting below the NAIRU and suffering from accelerating inflation (absent incomes policies). Training programs aimed at fighting structural unemployment would help here.
To the extent that hidden unemployment exists, it implies that official unemployment statistics provide a poor guide to what unemployment rate coincides with "full employment".
Even if everything was taken into account in a normal business environment in terms of public/private management of the economy; trace unemployement would occur due to human frailties of judgement towards others which occurs even today.