In general terms, an autogram is simply a sentence that is self-referential and provides information about itself in the content within it. A simple statement like "This sentence has only six words," is an example of such an expression. Much more complex forms of autogram can include a great deal of additional content describing the sentence, such as the number of specific letters and indications of punctuation used. The nature of these sentences also allows them to easily serve as a pangram, which is a sentence that includes at least one usage of every letter in a language.
An autogram typically contains grammatical information, or the sentence identifies information about word usage, though just about any type of data can be provided within it. Something as simple as "This sentence includes no adverbs," can be an autogram, though one could also be "This sentence is not a question." These are fairly simplistic forms, however, since they deal with general and easy information that is quickly identified by both the writer and reader.
A much more complicated type of autogram is a sentence that might begin, "This sentence contains only three a's, three c's, two d's, twenty-five e's;" the sentence goes on to identify the number of letters within it from A to Z. This becomes quite complicated, since each spelling of the numbers needs to be considered as part of the final letter count. Incidentally, the letter "s" also gains quite a high number of uses since each letter requires "s" to indicate it is a plural.
By its very nature, an autogram can also be used to create a pangram or a sentence that includes at least one use of each letter. For example, in English, the sentence "The quick brown fox jumps over the lazy dog," contains each letter in the alphabet. An autogram that is created as a pangram can simply indicate each letter that may otherwise be missing, and note a single usage of it. This is a somewhat artificial form of pangram, however, since some of the letters may simple be forced into the sentence.
Although not indicative of grammatical content or word choice, a sentence can also be created that is self-referential and paradoxical, such as "This statement is false." Logically, this sentence is meaningless. If it is false, then it would be true in saying so and cannot be false; if it is true, then it would be false, which cannot make it true.