What is claimed is:1. A method performed in a system for indicating a characteristic of a problem set including a plurality of problems, the method comprising the steps of:defining a plurality of attributes for expressing characteristics of the problems included in the problem set, wherein the plurality of defined attributes are stored in an attribute database included;determining characteristics of the problems included in the problem set, wherein the characteristic of each of the problems is determined by attributes related with each of the problems and correlations between the attributes;determining a characteristic of the problem set as a union of the characteristics of all the problems included in the problem set, wherein the characteristic of the problem set is determined by attributes related with all the problems and correlations between the attributes; andvisualizing the determined characteristic of the problem set on a screen,wherein the step of determining the characteristics of the problems comprises the steps of:identifying attributes related with each of the problems; andidentifying correlations between the identified attributes in case a number of the identified attributes is larger than one, andwherein the step of visualizing comprises:displaying a plurality of points corresponding to the attributes related with each of the problems;displaying lines, each of the lines connected between two points of the plurality of points and corresponding to a correlation between two identified attributes; anddynamically changing a thickness of each of the lines based on a frequency of the correlation corresponding to each of the lines being included in the determined characteristic of the problem set.2. The method according to claim 1, wherein in the step of determining the characteristics of the problems, the attributes related with each of the problems are selected as attributes related with each of the problems among the plurality of defined attributes.3. The method according to claim 1, wherein an appearance of a point corresponding to a specific attribute varies with a frequency of the specific attribute being included in the determined characteristic of the problem set.4. The method according to claim 1, wherein in the step of visualizing the determined characteristic of the problem set, attributes included in the determined characteristic of the problem set and correlations between the corresponding attributes are visualized on the screen in a sequential manner by the characteristic of each of the problems.5. The method according to claim 1, wherein in the step of visualizing the determined characteristic of the problem set, attributes included in the determined characteristic of the problem set and correlations between the corresponding attributes are visualized on the screen in an accumulative manner by the characteristic of each of the problems.6. The method according to claim 5, wherein an appearance of a point corresponding to a specific attribute varies with a frequency of the specific attribute being accumulated.7. The method according to claim 1, wherein in the step of visualizing the determined characteristic of the problem set, the attributes included in the determined characteristic of the problem set are indicated as points on the screen in locations predetermined for the corresponding attributes.8. The method according to claim 1, wherein in the step of visualizing the determined characteristic of the problem set, the attributes included in the determined characteristic of the problem set are indicated as points on the screen in variable locations.9. The method according to claim 1, wherein an appearance of a line corresponding to a correlation between attributes related with a problem to which a learner gives a right answer differs from an appearance of a line corresponding to a correlation between attributes related with a problem to which the learner gives a wrong answer.10. A method performed in a system for indicating a characteristic of a problem set including a plurality of problems, the method comprising the steps of:defining a plurality of attributes for expressing characteristics of the problems included in the problem set, wherein the plurality of defined attributes are stored in an attribute database;determining characteristics of the problems included in the problem set, wherein the characteristic of each of the problems is determined by attributes related with each of the problems and correlations between the attributes;determining a characteristic of the problem set as a union of the characteristics of all the problems included in the problem set, wherein the characteristic of the problem set is determined by attributes related with all the problems and correlations between the attributes; andvisualizing the determined characteristic of the problem set on a screen,wherein the step of determining the characteristics of the problems comprises the steps of:identifying attributes related with each of the problems; andidentifying correlations between the identified attributes in case a number of the identified attributes is larger than one, andwherein the step of visualizing comprises:displaying a plurality of points corresponding to the attributes related with each of the problems;displaying a first line connected between a first point and a second point among the plurality of points based on a correlation related to the characteristic of a first problem;displaying a second line connected between the first point and the second point based on a correlation related to the characteristic of a second problem;displaying the first line in a first color in response to a learner giving a right answer to the correlation related to the characteristic of the first problem; anddisplaying the second line in a second color different from the first color in response to the learner giving a wrong answer to the correlation related to the characteristic of the second problem.11. The method of claim 10, further comprising:dynamically changing a thickness of the first line in response to the learner giving a right answer to a correlation related to the characteristic of a third problem, the correlation related to the characteristic of the third problem being a correlation between an attribute corresponding to the first point and an attribute corresponding the second point.12. The method of claim 10, further comprising:dynamically changing a thickness of the second line in response to the learner giving a wrong answer to a correlation related to the characteristic of a third problem, the correlation related to the characteristic of the third problem being a correlation between an attribute corresponding to the first point and an attribute corresponding the second point.