Further to @dust42, BERT is an encoder, GPT is a decoder, and T5 is an encoder-decoder.

Encoder-decoders are not in vogue.

Encoders are favored for classification, extraction (eg, NER and extractive QA) and information retrieval.

Decoders are favored for text generation, summarization and translation.

Recent research (see, eg, the Ettin paper: https://arxiv.org/html/2507.11412v1 ) seems to confirm the previous understanding that encoders are indeed better for “encoder task” and vice-versa.

Fundamentally, both are transformers and so an encoder could be turned into a decoder or a decoder could be turned into an encoder.

The design difference comes down to bidirectional (ie, all tokens can attend to all other tokens) versus autoregressive attention (ie, the current token can only attend to the previous tokens).

You can use an encoder style architecture with decoder style output heads up top for denoising diffusion mode mask/blank filling. They seem to be somewhat more expensive on short sequences than GPT style decoder-only models when you batch them, as you need fewer passes over the content and until sequence length blows up your KV cache throughout cost, fewer passes are cheaper. But for situations that don't get request batching or where the context length is so heavy that you'd prefer to get to exploit memory locality on the attention computation, you'd benefit from diffusion mode decoding.

A nice side effect of the diffusion mode is that it's natural reliance on the bidirectional attention from the encoder layers provides much more flexible (and, critically, context-aware) understanding so as mentioned, later words can easily modulate earlier words like with "bank [of the river]"/"bank [in the park]"/"bank [got robbed]" or the classic of these days: telling an agent it did wrong and expecting it to in-context learn from the mistake (in practice decoder-only models basically merely get polluted from that, so you have to re-wind the conversation, because the later correction has literally no way of backwards-affecting the problematic tokens).

That said, the recent surge in training "reasoning" models to utilize thinking tokens that often get cut out of further conversation context, and all via a reinforcement learning process that's not merely RLHF/preference-conditioning, is actually quite related: discrete denoising diffusion models can be trained as a RL scheme during pre training where the training step is provided the outcome goal and a masked version as the input query, and then trained to manage the work done in the individual steps on it's own to where it eventually produces the outcome goal, crucially without prescribing any order of filling in the masked tokens or how many to do in which step.

A recent paper on the matter: https://openreview.net/forum?id=MJNywBdSDy

Until we got highly optimized decoder implementations, decoders for prefill were often even implemented by using the same implementation as an encoder, but logit-masking inputs using a causal mask before the attention softmax so that tokens could not attend to future tokens.